feat(PROJ-53): Konfigurierbare Listenanzahl pro Seite

- users.list_page_size (Default 25), PATCH /api/auth/preferences,
  Whitelist 25/50/100/200, Wert in login/me-Response
- Settings-UI mit Select, /search nutzt gespeicherte Seitengröße
- /api/search page_size serverseitig auf max. 500 gecappt

fix(PROJ-46): login_attempts-Migration nutzte s.db statt s.pool
(Backend kompilierte nicht)

feat(PROJ-50): DSGVO-Löschersuchen Backend (dsgvo_requests, Handler,
cc_addr/bcc_addr Indexerweiterung) — noch nicht QA'd/deployed

internal/api/profile_handlers.go

@@ -150,3 +150,45 @@ func (s *Server) handleChangeEmail(w http.ResponseWriter, r *http.Request) {
 
 	writeJSON(w, http.StatusOK, map[string]interface{}{"ok": true, "email": req.Email})
 }
+
+// allowedListPageSizes are the valid values for users.list_page_size (PROJ-53).
+var allowedListPageSizes = map[int]bool{25: true, 50: true, 100: true, 200: true}
+
+// handleChangePreferences allows a user to change their own UI preferences,
+// currently just the number of list entries shown per page.
+// PATCH /api/auth/preferences
+func (s *Server) handleChangePreferences(w http.ResponseWriter, r *http.Request) {
+	sess := sessionFromCtx(r.Context())
+	if sess.UserID == 0 {
+		writeError(w, http.StatusUnauthorized, "not authenticated")
+		return
+	}
+
+	var req struct {
+		ListPageSize int `json:"list_page_size"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if !allowedListPageSizes[req.ListPageSize] {
+		writeError(w, http.StatusBadRequest, "invalid list_page_size: must be 25, 50, 100 or 200")
+		return
+	}
+
+	user, err := s.users.GetByUsername(sess.Username)
+	if err != nil {
+		s.logger.Error("change_preferences: user not found", "err", err, "username", sess.Username)
+		writeError(w, http.StatusInternalServerError, "user not found")
+		return
+	}
+
+	if err := s.users.UpdateListPageSize(r.Context(), user.ID, req.ListPageSize); err != nil {
+		s.logger.Error("change_preferences: update failed", "err", err)
+		writeError(w, http.StatusInternalServerError, "failed to update preferences")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, map[string]interface{}{"ok": true, "list_page_size": req.ListPageSize})
+}