Verify login credentials before spending rate-limit budget (closes #150)

With TRUSTED_PROXIES empty behind the reverse proxy production is
required to run behind, every login POST keyed on the proxy's address
and shared one 5/minute bucket. A stranger sending five POSTs a
minute -- 0.08 requests per second, from anywhere -- kept that bucket
permanently full, and the operator's own correct password was answered
429 indefinitely with no second administrative path.

The login POST no longer has a pre-emptive limiter. The handler
verifies credentials first and spends budget only on a FAILED attempt,
so a correct password is never throttled whatever the counters hold.
Three things follow, and are implemented together because the first is
unsafe without the other two:

- Failures are counted per (client bucket, submitted username), five
  per minute, after which further failures get 429 with a Retry-After.
  A successful login clears the counter, so mistyping and then
  succeeding does not leave the operator throttled.
- Both key sets are capped at 1024 entries. The submitted username is
  attacker-controlled, so past the first cap failures fall back to a
  counter keyed on the client alone, and past both caps a failure is
  answered as throttled without being recorded. Tracked state stays
  under half a megabyte and does not grow with invented usernames.
- Concurrent Argon2id verifications are capped at two, a 128 MB
  ceiling at 64 MB per hash, and the queue for those slots is capped
  at 16 waiters. Every password-hashing endpoint takes a slot,
  including the password-change endpoint, which holds one across both
  its hashes. A request that waits five seconds without a slot is
  answered 503, and one that arrives with the queue already full is
  shed with 503 immediately rather than joining it. Bounding the wait
  alone would not bound memory, and the queue depth is sized from what
  a parked waiter measurably retains rather than from the 1 MB body
  cap, which bounds only the raw body read. The body-cap, CSRF and
  form-parsing middleware all run before the guard, so a waiter holds
  its parsed form plus its request header block for the whole wait.
  Measured on the pinned go1.26.1 toolchain as the HeapAlloc delta
  across two GCs with 64 waiters parked in the handler: an ordinary
  two-field login form retains ~0 MB, a 1 MB urlencoded body at Go's
  10,000-parameter parse cap retains 2.82 MB (3.09 MB with %41
  escapes), and the ~0.9 MB of headers the 1 MB header cap allows
  takes it to 4.18 MB. The retained parse and the header block
  dominate, not the raw body. So 16 waiters: 16 x 4.18 MB is about
  67 MB of committed queue memory, and two slots drain a full 16-deep
  queue in about 0.6 s, far inside the deadline. Peak commitment for
  the endpoint is about 203 MB — 128 MB of Argon2id plus the 18
  requests holding a parsed form, 16 queued and the 2 being hashed, at
  about 75 MB.

An unknown username is verified against a dummy hash instead of
returning early, so a nonexistent account costs the same time as a
real one and the response cannot be used to enumerate usernames.

The password-change limiter is unchanged: RequireAuth runs ahead of
it, so only a request already carrying a valid session reaches its
bucket.

Two consequences are documented rather than fixed, because they follow
from the shape the issue asks for. Online guessing throughput rises
from 5 a minute to roughly 27 a second, about 2.3 million a day: the
credential check always precedes the counter, so the 429 is a label on
the response rather than a gate in front of the hash, and what bounds
brute force is the semaphore. And under a sustained flood the residual
exposure is a loss of login availability, not merely of latency --
above about 27 requests a second most attempts are shed with 503, so a
determined flood still denies login for as long as it runs. It costs
roughly 400x more to run, nothing accumulates, and the first attempt
after it stops succeeds. Restarting the service does not help: the
counters a restart clears are not what is saturated.

Also adds the missing test for the third bucketKey call site, where
the peer is a trusted proxy but the forwarded chain names no client.
Every existing test of that fallback uses an IPv4 proxy, where
bucketKey is the identity function, so dropping the /64 masking there
left the suite green.

README and the TRUSTED_PROXIES startup warning updated: a shared
bucket now costs precision, not the availability of the admin path.

internal/middleware/ratelimit_test.go

@@ -20,14 +20,14 @@ import (
 	"sneak.berlin/go/webhooker/internal/middleware"
 )
 
-func TestLoginRateLimit_AllowsGET(t *testing.T) {
+func TestPostRateLimit_AllowsGET(t *testing.T) {
 	t.Parallel()
 
 	m, _ := testMiddleware(t, config.EnvironmentDev)
 
 	var callCount int
 
-	handler := m.LoginRateLimit()(http.HandlerFunc(
+	handler := m.PasswordChangeRateLimit()(http.HandlerFunc(
 		func(w http.ResponseWriter, _ *http.Request) {
 			callCount++
 
@@ -39,7 +39,7 @@ func TestLoginRateLimit_AllowsGET(t *testing.T) {
 	for i := range 20 {
 		req := httptest.NewRequestWithContext(
 			context.Background(),
-			http.MethodGet, "/pages/login", nil,
+			http.MethodGet, "/user/admin/password", nil,
 		)
 		req.RemoteAddr = "192.168.1.1:12345"
 
@@ -110,20 +110,6 @@ func runPostLimitTest(
 	assert.Equal(t, limit, callCount)
 }
 
-func TestLoginRateLimit_LimitsPOST(t *testing.T) {
-	t.Parallel()
-
-	m, _ := testMiddleware(t, config.EnvironmentDev)
-
-	runPostLimitTest(
-		t,
-		m.LoginRateLimit(),
-		middleware.LoginRateLimitConst,
-		"/pages/login",
-		"10.0.0.1:12345",
-	)
-}
-
 func TestPasswordChangeRateLimit_LimitsPOST(t *testing.T) {
 	t.Parallel()
 
@@ -138,19 +124,19 @@ func TestPasswordChangeRateLimit_LimitsPOST(t *testing.T) {
 	)
 }
 
-func TestLoginRateLimit_IndependentPerIP(t *testing.T) {
+func TestPostRateLimit_IndependentPerIP(t *testing.T) {
 	t.Parallel()
 
 	m, _ := testMiddleware(t, config.EnvironmentDev)
 
-	handler := m.LoginRateLimit()(http.HandlerFunc(
+	handler := m.PasswordChangeRateLimit()(http.HandlerFunc(
 		func(w http.ResponseWriter, _ *http.Request) {
 			w.WriteHeader(http.StatusOK)
 		},
 	))
 
 	// Exhaust limit for IP1
-	for range middleware.LoginRateLimitConst {
+	for range middleware.PasswordChangeRateLimitConst {
 		req := httptest.NewRequestWithContext(
 			context.Background(),
 			http.MethodPost, "/pages/login", nil,
@@ -367,7 +353,14 @@ func TestReceiverRateLimit_CountsEveryMethod(t *testing.T) {
 }
 
 const (
-	loginPath  = "/pages/login"
+	// limitedPath is the endpoint these tests drive the shared POST
+	// rate limiter through. It is the password-change path: since
+	// the login POST verifies credentials before spending any
+	// budget, the password-change limiter is the only pre-emptive
+	// POST limiter left, and it is what pins the shared key
+	// function's behaviour here.
+	limitedPath = "/user/admin/password"
+
 	headerXFF  = "X-Forwarded-For"
 	headerReal = "X-Real-IP"
 	headerTrue = "True-Client-IP"
@@ -415,10 +408,10 @@ func assertSharedBucket(
 	m := rateLimitMiddleware(
 		t, &config.Config{TrustedProxies: proxies},
 	)
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
-	for i := range middleware.LoginRateLimitConst {
-		w := postWithHeaders(handler, peer, loginPath, headers(i))
+	for i := range middleware.PasswordChangeRateLimitConst {
+		w := postWithHeaders(handler, peer, limitedPath, headers(i))
 		assert.Equal(
 			t, http.StatusOK, w.Code,
 			"request %d should pass", i,
@@ -426,8 +419,8 @@ func assertSharedBucket(
 	}
 
 	w := postWithHeaders(
-		handler, peer, loginPath,
-		headers(middleware.LoginRateLimitConst),
+		handler, peer, limitedPath,
+		headers(middleware.PasswordChangeRateLimitConst),
 	)
 	assert.Equal(t, http.StatusTooManyRequests, w.Code, msg)
 }
@@ -549,24 +542,24 @@ func TestRateLimitKey_ForwardedHonouredFromTrustedPeer(
 	m := rateLimitMiddleware(t, &config.Config{
 		TrustedProxies: trustedProxies(trustedProxyCIDR),
 	})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
 	const peer = trustedPeer
 
 	first := map[string]string{headerXFF: clientIPv4}
 
-	for range middleware.LoginRateLimitConst {
-		postWithHeaders(handler, peer, loginPath, first)
+	for range middleware.PasswordChangeRateLimitConst {
+		postWithHeaders(handler, peer, limitedPath, first)
 	}
 
-	w := postWithHeaders(handler, peer, loginPath, first)
+	w := postWithHeaders(handler, peer, limitedPath, first)
 	assert.Equal(
 		t, http.StatusTooManyRequests, w.Code,
 		"the forwarded client's own bucket must fill up",
 	)
 
 	w = postWithHeaders(
-		handler, peer, loginPath,
+		handler, peer, limitedPath,
 		map[string]string{headerXFF: clientIPv4Alt},
 	)
 	assert.Equal(
@@ -662,7 +655,7 @@ func TestRateLimitKey_LongChainAllocationIsBounded(t *testing.T) {
 	})
 
 	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodPost, loginPath, nil,
+		context.Background(), http.MethodPost, limitedPath, nil,
 	)
 	req.RemoteAddr = trustedPeer
 	req.Header.Set(
@@ -869,7 +862,7 @@ func clientKeyFor(
 	t.Helper()
 
 	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodPost, loginPath, nil,
+		context.Background(), http.MethodPost, limitedPath, nil,
 	)
 	req.RemoteAddr = remoteAddr
 
@@ -1019,23 +1012,23 @@ func TestRateLimitKey_UnparseablePeerKeepsDistinctBuckets(
 	)
 }
 
-// TestLoginRateLimit_IPv6SharesBucketWithinSlash64 is the behavioural
+// TestPostRateLimit_IPv6SharesBucketWithinSlash64 is the behavioural
 // half, and the regression test for the bypass itself: a client that
 // rotates source addresses inside its own routed /64 must stay in one
 // bucket. Reverting the masking makes this test fail, because each
 // rotated address would mint a fresh bucket and nothing would be
 // rejected.
-func TestLoginRateLimit_IPv6SharesBucketWithinSlash64(t *testing.T) {
+func TestPostRateLimit_IPv6SharesBucketWithinSlash64(t *testing.T) {
 	t.Parallel()
 
 	m := rateLimitMiddleware(t, &config.Config{})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
-	for i := range middleware.LoginRateLimitConst {
+	for i := range middleware.PasswordChangeRateLimitConst {
 		w := postWithHeaders(
 			handler,
 			fmt.Sprintf("[2001:db8:1:2::%d]:44444", i+1),
-			loginPath, nil,
+			limitedPath, nil,
 		)
 		assert.Equal(
 			t, http.StatusOK, w.Code, "request %d should pass", i,
@@ -1043,7 +1036,7 @@ func TestLoginRateLimit_IPv6SharesBucketWithinSlash64(t *testing.T) {
 	}
 
 	w := postWithHeaders(
-		handler, "[2001:db8:1:2::ffff]:44444", loginPath, nil,
+		handler, "[2001:db8:1:2::ffff]:44444", limitedPath, nil,
 	)
 	assert.Equal(
 		t, http.StatusTooManyRequests, w.Code,
@@ -1052,23 +1045,23 @@ func TestLoginRateLimit_IPv6SharesBucketWithinSlash64(t *testing.T) {
 	)
 }
 
-// TestLoginRateLimit_IPv6IndependentAcrossSlash64 is the other side
+// TestPostRateLimit_IPv6IndependentAcrossSlash64 is the other side
 // of the trade: bucketing by /64 must not merge separate allocations,
 // so a client in a different /64 keeps its own limit.
-func TestLoginRateLimit_IPv6IndependentAcrossSlash64(t *testing.T) {
+func TestPostRateLimit_IPv6IndependentAcrossSlash64(t *testing.T) {
 	t.Parallel()
 
 	m := rateLimitMiddleware(t, &config.Config{})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
-	for range middleware.LoginRateLimitConst + 1 {
+	for range middleware.PasswordChangeRateLimitConst + 1 {
 		postWithHeaders(
-			handler, "[2001:db8:1:2::1]:44444", loginPath, nil,
+			handler, "[2001:db8:1:2::1]:44444", limitedPath, nil,
 		)
 	}
 
 	w := postWithHeaders(
-		handler, "[2001:db8:1:3::1]:44444", loginPath, nil,
+		handler, "[2001:db8:1:3::1]:44444", limitedPath, nil,
 	)
 	assert.Equal(
 		t, http.StatusOK, w.Code,
@@ -1076,23 +1069,23 @@ func TestLoginRateLimit_IPv6IndependentAcrossSlash64(t *testing.T) {
 	)
 }
 
-// TestLoginRateLimit_IPv4IndependentPerAddress guards against the
+// TestPostRateLimit_IPv4IndependentPerAddress guards against the
 // masking leaking into IPv4: two addresses one apart must still hold
 // separate buckets.
-func TestLoginRateLimit_IPv4IndependentPerAddress(t *testing.T) {
+func TestPostRateLimit_IPv4IndependentPerAddress(t *testing.T) {
 	t.Parallel()
 
 	m := rateLimitMiddleware(t, &config.Config{})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
-	for range middleware.LoginRateLimitConst + 1 {
+	for range middleware.PasswordChangeRateLimitConst + 1 {
 		postWithHeaders(
-			handler, clientIPv4+":44444", loginPath, nil,
+			handler, clientIPv4+":44444", limitedPath, nil,
 		)
 	}
 
 	w := postWithHeaders(
-		handler, clientIPv4Alt+":44444", loginPath, nil,
+		handler, clientIPv4Alt+":44444", limitedPath, nil,
 	)
 	assert.Equal(
 		t, http.StatusOK, w.Code,
@@ -1112,7 +1105,7 @@ func forwardedKeyFor(
 	t.Helper()
 
 	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodPost, loginPath, nil,
+		context.Background(), http.MethodPost, limitedPath, nil,
 	)
 	req.RemoteAddr = trustedPeer
 	req.Header.Set(headerXFF, forwarded)
@@ -1177,11 +1170,77 @@ func TestRateLimitKey_ForwardedIPv6BucketsByPrefix(t *testing.T) {
 	}
 }
 
-// TestLoginRateLimit_ForwardedIPv6SharesBucketWithinSlash64 is the
+// TestRateLimitKey_TrustedPeerUnusableForwardedMasksPeer covers the
+// third bucketKey call site: the peer IS a trusted proxy, but the
+// forwarded chain cannot name a client, so the key falls back to the
+// peer address — and that fallback owes the same /64 masking every
+// other key gets.
+//
+// Every existing test of this fallback uses an IPv4 proxy, where
+// bucketKey is the identity function, so replacing the call with
+// peer.String() leaves the whole suite green. Only operator-listed
+// addresses reach this line and the fallback is fail-closed, so this
+// pins behaviour rather than fixing a defect.
+func TestRateLimitKey_TrustedPeerUnusableForwardedMasksPeer(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	const (
+		proxyCIDR = "2001:db8:ffff::/48"
+		proxyPeer = "[2001:db8:ffff:1::5]:44444"
+		wantKey   = "2001:db8:ffff:1::/64"
+	)
+
+	m := rateLimitMiddleware(t, &config.Config{
+		TrustedProxies: trustedProxies(proxyCIDR),
+	})
+
+	for _, tc := range []struct {
+		name      string
+		forwarded string
+		about     string
+	}{{
+		name:  "absent",
+		about: "no X-Forwarded-For at all falls back to the peer",
+	}, {
+		name:      "unreadable-hop",
+		forwarded: "unknown",
+		about: "a hop that is not a bare address ends the walk " +
+			"and falls back to the peer",
+	}, {
+		name:      "all-hops-trusted",
+		forwarded: "2001:db8:ffff:2::9",
+		about: "a chain naming only trusted proxies names no " +
+			"client, so the peer is used",
+	}} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			req := httptest.NewRequestWithContext(
+				context.Background(),
+				http.MethodPost, limitedPath, nil,
+			)
+			req.RemoteAddr = proxyPeer
+
+			if tc.forwarded != "" {
+				req.Header.Set(headerXFF, tc.forwarded)
+			}
+
+			assert.Equal(
+				t, wantKey,
+				middleware.ClientKeyForTest(m, req),
+				"%s, masked to its /64", tc.about,
+			)
+		})
+	}
+}
+
+// TestPostRateLimit_ForwardedIPv6SharesBucketWithinSlash64 is the
 // behavioural half on the production path: behind a trusted proxy, a
 // client rotating source addresses inside its own routed /64 must
 // stay in one bucket.
-func TestLoginRateLimit_ForwardedIPv6SharesBucketWithinSlash64(
+func TestPostRateLimit_ForwardedIPv6SharesBucketWithinSlash64(
 	t *testing.T,
 ) {
 	t.Parallel()
@@ -1198,10 +1257,10 @@ func TestLoginRateLimit_ForwardedIPv6SharesBucketWithinSlash64(
 	)
 }
 
-// TestLoginRateLimit_ForwardedIPv6IndependentAcrossSlash64 is the
+// TestPostRateLimit_ForwardedIPv6IndependentAcrossSlash64 is the
 // other side of that trade on the same path: bucketing by /64 must
 // not merge two allocations reaching the proxy.
-func TestLoginRateLimit_ForwardedIPv6IndependentAcrossSlash64(
+func TestPostRateLimit_ForwardedIPv6IndependentAcrossSlash64(
 	t *testing.T,
 ) {
 	t.Parallel()
@@ -1209,15 +1268,15 @@ func TestLoginRateLimit_ForwardedIPv6IndependentAcrossSlash64(
 	m := rateLimitMiddleware(t, &config.Config{
 		TrustedProxies: trustedProxies(trustedProxyCIDR),
 	})
-	handler := m.LoginRateLimit()(okHandler())
+	handler := m.PasswordChangeRateLimit()(okHandler())
 
 	spent := map[string]string{headerXFF: clientIPv6}
-	for range middleware.LoginRateLimitConst + 1 {
-		postWithHeaders(handler, trustedPeer, loginPath, spent)
+	for range middleware.PasswordChangeRateLimitConst + 1 {
+		postWithHeaders(handler, trustedPeer, limitedPath, spent)
 	}
 
 	w := postWithHeaders(
-		handler, trustedPeer, loginPath,
+		handler, trustedPeer, limitedPath,
 		map[string]string{headerXFF: clientIPv6Other},
 	)
 	assert.Equal(