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/handlers_auth.go

@@ -168,6 +168,10 @@ func (s *Server) handleRegister(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusBadRequest, "Ungültige Anfrage")
 		return
 	}
+	if wait := s.loginGuard.throttle("register", clientIPFrom(r), limitRegisterPerIP); wait > 0 {
+		writeThrottled(w, wait)
+		return
+	}
 	if strings.TrimSpace(req.Email) == "" {
 		writeError(w, http.StatusBadRequest, "E-Mail-Adresse erforderlich")
 		return
@@ -212,15 +216,26 @@ func (s *Server) handleRegister(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// The invite is claimed before the account exists, so it cannot be redeemed twice at once
+	var claim string
+	if validInvite != nil {
+		if claim, err = s.store.ClaimUserInvite(validInvite.ID); err != nil {
+			writeError(w, http.StatusForbidden, err.Error())
+			return
+		}
+	}
+
 	user, err := s.store.CreateUser(req.Email, hash, req.DisplayName, role)
 	if err != nil {
+		if validInvite != nil {
+			s.store.ReleaseUserInvite(validInvite.ID, claim)
+		}
 		writeError(w, http.StatusConflict, "E-Mail-Adresse existiert bereits")
 		return
 	}
 
 	if validInvite != nil {
-		// Otherwise the code could be used again
-		if err := s.store.MarkUserInviteUsed(validInvite.ID, user.ID); err != nil {
+		if err := s.store.FinishUserInvite(validInvite.ID, claim, user.ID); err != nil {
 			slog.Error("Einladungscode konnte nicht als benutzt markiert werden", "invite", validInvite.ID, "error", err)
 		}
 		if validInvite.OrganizationID != "" {