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

In the shipped default, any stranger denied the operator the only
administrative path at 5 requests per minute: TRUSTED_PROXIES is empty,
the README requires a reverse proxy, so every login POST shared one
bucket keyed on the proxy.

Credentials are now verified first and only a FAILED attempt spends
budget, so a correct password is never throttled. Failures are counted
per (client bucket, submitted username), bounded. Concurrent Argon2id
verifications are capped at two, and the queue for them at 16 — because
verifying first lets an attacker force a 64 MB hash per request, and
bounding the wait alone bounds nothing.

The issue's own recommendation was insufficient and is rejected here:
keying by username stops an attacker locking out a DIFFERENT account,
but this is a single-admin product with a predictable bootstrap
username, so flooding the operator's own name still locks them out.

This is speculative — it implements a corrected recommendation ahead of
the owner's ruling so the decision can be made by merging or reverting.
Three things are disclosed rather than glossed: online guessing rises
from 5/min to roughly 27/s, because the 429 is a label on the response
and not a gate in front of the hash; the residual exposure is a loss of
login AVAILABILITY, not latency, and a determined flood still denies
login while it runs, at ~400x the cost and clearing the moment it
stops; and the endpoint should be provisioned for ~400 MB resident, not
the 203 MB of live commitment it itemises.

Independently reviewed four times. Reviewers disproved the suspected
FIFO starvation by measurement, then caught two successive memory
bounds the code did not have — the second by parking waiters and
reading the heap rather than checking the arithmetic.

internal/handlers/auth.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"net/http"
+	"strconv"
 
 	"sneak.berlin/go/webhooker/internal/database"
 )
@@ -93,6 +94,16 @@ func (h *Handlers) renderLoginError(
 
 // authenticateUser looks up and verifies a user's credentials.
 // On failure it writes an HTTP response and returns an error.
+//
+// The credential check runs BEFORE any rate-limit budget is
+// consulted, and only a failed check spends budget. That is what
+// keeps the single administrative path reachable: behind the reverse
+// proxy this deployment requires, with TRUSTED_PROXIES unset, every
+// client shares one bucket, so a limiter spent on arrival lets any
+// stranger deny the operator's own correct password indefinitely.
+//
+// Verifying first means every login POST costs an Argon2id hash, so
+// the work is taken under a bounded number of verification slots.
 func (h *Handlers) authenticateUser(
 	w http.ResponseWriter,
 	r *http.Request,
@@ -100,16 +111,37 @@ func (h *Handlers) authenticateUser(
 ) (database.User, error) {
 	var user database.User
 
+	release, ok := h.mw.BeginPasswordVerification(r.Context())
+	if !ok {
+		h.log.Warn(
+			"password verification capacity exhausted",
+			"path", r.URL.Path,
+		)
+		h.renderLoginError(
+			w, r,
+			"The server is busy verifying credentials. "+
+				"Please try again.",
+			http.StatusServiceUnavailable,
+		)
+
+		return user, errVerificationBusy
+	}
+
+	defer release()
+
 	err := h.db.DB().Where(
 		"username = ?", username,
 	).First(&user).Error
 	if err != nil {
+		// A username that does not exist is charged the same work
+		// as one that does. Skipping the hash here would answer in
+		// microseconds where a real account takes tens of
+		// milliseconds, handing every client a username oracle.
+		h.dummyVerifications.Add(1)
+		database.VerifyDummyPassword(password)
+
 		h.log.Debug("user not found", "username", username)
-		h.renderLoginError(
-			w, r,
-			"Invalid username or password",
-			http.StatusUnauthorized,
-		)
+		h.rejectLogin(w, r, username)
 
 		return user, err
 	}
@@ -127,16 +159,49 @@ func (h *Handlers) authenticateUser(
 
 	if !valid {
 		h.log.Debug("invalid password", "username", username)
+		h.rejectLogin(w, r, username)
+
+		return user, errInvalidPassword
+	}
+
+	// The password was correct, so forgive whatever failures this
+	// client accumulated: an operator who mistypes a few times and
+	// then gets it right must not stay throttled afterwards.
+	h.mw.ForgiveLoginFailures(r, username)
+
+	return user, nil
+}
+
+// rejectLogin counts one failed credential verification and answers
+// it: 401 while this client still has failure budget against the
+// submitted username, 429 with a Retry-After once it is spent.
+//
+// The 429 throttles wrong passwords only. A correct one never
+// reaches here, so no amount of failure — from this client or any
+// other sharing its bucket — can keep the operator out.
+func (h *Handlers) rejectLogin(
+	w http.ResponseWriter,
+	r *http.Request,
+	username string,
+) {
+	if !h.mw.RecordLoginFailure(r, username) {
 		h.renderLoginError(
 			w, r,
 			"Invalid username or password",
 			http.StatusUnauthorized,
 		)
 
-		return user, errInvalidPassword
+		return
 	}
 
-	return user, nil
+	w.Header().Set("Retry-After", strconv.Itoa(int(
+		h.mw.LoginFailureInterval().Seconds(),
+	)))
+	h.renderLoginError(
+		w, r,
+		"Too many failed login attempts. Please try again later.",
+		http.StatusTooManyRequests,
+	)
 }
 
 // createAuthenticatedSession regenerates the session and stores