feat(auth): lock logins after repeated failures, trust proxies explicitly (FLAW-09)

Failed logins are counted per account+address (3), per account (20) and
per address (30) within 15 minutes; a full bucket answers 429 with
Retry-After for 15 minutes. Locking the account+address bucket first
means an attacker cannot lock the owner out from elsewhere. Unknown
names count like accounts. Success clears the account buckets, a
password reset (mail or admin) lifts the lock. LOGIN_PROTECTION=false
switches it off.

chi's RealIP believed X-Forwarded-For from anyone. The client address
now comes from the headers only when the connection is from a trusted
proxy: TRUSTED_PROXIES=private (default: loopback, Docker, LAN), all
(old behaviour), none, or CIDRs. A proxy that sends no client address
is logged once and per-address limits are skipped for it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/handlers_admin.go

@@ -217,6 +217,7 @@ func (s *Server) handleAdminResetPassword(w http.ResponseWriter, r *http.Request
 		writeError(w, http.StatusInternalServerError, "Passwort-Hash fehlgeschlagen")
 		return
 	}
+	s.loginGuard.unlockAccount(targetUserID)
 	if err := s.store.UpdateUserPassword(targetUserID, hash); err != nil {
 		writeError(w, http.StatusInternalServerError, err.Error())
 		return