Rate-limit the password change endpoint (#65)

The password change POST verifies the current password, making it
a password-based authentication endpoint that REPO_POLICIES.md
requires rate limiting on. Extract the login limiter's POST-only
per-IP pattern into a shared postRateLimit helper and apply the
same 5-per-minute limit to POST /user/{username}/password.

internal/middleware/ratelimit.go

@@ -14,6 +14,16 @@ const (
 
 	// loginRateInterval is the time window for the rate limit.
 	loginRateInterval = 1 * time.Minute
+
+	// passwordChangeRateLimit is the maximum number of password
+	// change attempts per interval. Each attempt verifies the
+	// current password, so the endpoint must be rate-limited
+	// like any other password-based authentication endpoint.
+	passwordChangeRateLimit = 5
+
+	// passwordChangeRateInterval is the time window for the
+	// password change rate limit.
+	passwordChangeRateInterval = 1 * time.Minute
 )
 
 // LoginRateLimit returns middleware that enforces per-IP rate
@@ -24,19 +34,53 @@ const (
 // honours X-Forwarded-For, X-Real-IP, and True-Client-IP headers
 // for reverse-proxy setups.
 func (m *Middleware) LoginRateLimit() func(http.Handler) http.Handler {
-	limiter := httprate.Limit(
+	return m.postRateLimit(
 		loginRateLimit,
 		loginRateInterval,
+		"login rate limit exceeded",
+		"Too many login attempts. Please try again later.",
+	)
+}
+
+// PasswordChangeRateLimit returns middleware that enforces
+// per-IP rate limiting on password change attempts. The change
+// endpoint verifies the current password, so without a limit a
+// stolen session could be used to brute-force it; the limit
+// matches the login endpoint's.
+func (m *Middleware) PasswordChangeRateLimit() func(http.Handler) http.Handler {
+	return m.postRateLimit(
+		passwordChangeRateLimit,
+		passwordChangeRateInterval,
+		"password change rate limit exceeded",
+		"Too many password change attempts. "+
+			"Please try again later.",
+	)
+}
+
+// postRateLimit builds middleware that enforces a per-IP rate
+// limit on POST requests only; all other methods pass through
+// unaffected. Requests over the limit receive a 429 with the
+// given response message, and each rejection is logged with the
+// given log message. IP extraction honours X-Forwarded-For,
+// X-Real-IP, and True-Client-IP headers for reverse-proxy
+// setups.
+func (m *Middleware) postRateLimit(
+	limit int,
+	interval time.Duration,
+	logMessage, responseMessage string,
+) func(http.Handler) http.Handler {
+	limiter := httprate.Limit(
+		limit,
+		interval,
 		httprate.WithKeyFuncs(httprate.KeyByRealIP),
 		httprate.WithLimitHandler(http.HandlerFunc(
 			func(w http.ResponseWriter, r *http.Request) {
-				m.log.Warn("login rate limit exceeded",
+				m.log.Warn(logMessage,
 					"path", r.URL.Path,
 				)
 				http.Error(
 					w,
-					"Too many login attempts. "+
-						"Please try again later.",
+					responseMessage,
 					http.StatusTooManyRequests,
 				)
 			},
@@ -50,8 +94,7 @@ func (m *Middleware) LoginRateLimit() func(http.Handler) http.Handler {
 			w http.ResponseWriter,
 			r *http.Request,
 		) {
-			// Only rate-limit POST requests (actual login
-			// attempts)
+			// Only rate-limit POST requests.
 			if r.Method != http.MethodPost {
 				next.ServeHTTP(w, r)