Honour forwarded headers only from trusted proxies (closes #259)

A new ClientIP middleware sets request.RemoteAddr to the client's
address before the request is logged. X-Forwarded-For and X-Real-IP
count only when the peer is in TRUSTED_PROXIES, which defaults to the
RFC 1918 ranges. The login rate limit, the session's IP address and
hostname, and the request log all use that address.

Model: opus-5-5

internal/handlers/api_test.go

@@ -4281,6 +4281,67 @@ func TestLoginRateLimitExceeded(t *testing.T) {
 	}
 }
 
+func TestLoginRateLimitIgnoresForwardedHeaders(t *testing.T) {
+	t.Parallel()
+
+	// The test client connects from 127.0.0.1, which is outside the
+	// default trusted proxies, so the server must ignore these headers.
+	tests := []struct {
+		name   string
+		header string
+	}{
+		{name: "X-Forwarded-For", header: "X-Forwarded-For"},
+		{name: "X-Real-IP", header: "X-Real-IP"},
+	}
+
+	loginBody, err := json.Marshal(map[string]string{
+		nickKey: "nosuchuser", passwordKey: "doesnotmatter",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			tserver := newTestServer(t)
+
+			// The default burst is 5 attempts, so the sixth gets 429
+			// although each claims a new address.
+			var status int
+
+			for attempt := range 6 {
+				request, rErr := http.NewRequestWithContext(
+					t.Context(), http.MethodPost,
+					tserver.url("/api/v1/login"),
+					bytes.NewReader(loginBody),
+				)
+				if rErr != nil {
+					t.Fatal(rErr)
+				}
+
+				request.Header.Set("Content-Type", "application/json")
+				request.Header.Set(
+					test.header, fmt.Sprintf("203.0.113.%d", attempt+1),
+				)
+
+				resp, dErr := http.DefaultClient.Do(request)
+				if dErr != nil {
+					t.Fatal(dErr)
+				}
+
+				status = resp.StatusCode
+				_ = resp.Body.Close()
+			}
+
+			if status != http.StatusTooManyRequests {
+				t.Fatalf("sixth attempt got %d, want 429", status)
+			}
+		})
+	}
+}
+
 func TestLoginRateLimitAllowsNormalUse(t *testing.T) {
 	t.Parallel()