fix(auth): longer IDs and invite codes, register limits, single-use invites (FLAW-32, FLAW-39, FLAW-40)

- NewID uses 96 random bits; invite codes get their own 100 bits in Base32.
- Registration and forgot-password are limited per address via loginGuard.
- The invite is claimed atomically before the account is created and
  released again when the account cannot be created.

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

internal/api/password_reset.go

@@ -29,6 +29,10 @@ func (s *Server) handleForgotPassword(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusServiceUnavailable, "Passwort-Zurücksetzen ist auf diesem Server nicht eingerichtet. Bitte wende dich an die Administration.")
 		return
 	}
+	if wait := s.loginGuard.throttle("forgot", clientIPFrom(r), limitForgotPerIP); wait > 0 {
+		writeThrottled(w, wait)
+		return
+	}
 	var req struct {
 		Email string `json:"email"`
 	}