feat: add customizable event types with admin management

Implements GitHub issue #139 - allows users to create and manage custom
event types beyond the default presets (wedding, birthday, corporate, other).

Backend:
- Add event_types table migration with default system types
- Create eventTypeService for CRUD operations with legacy fallback
- Add adminEventTypes routes with full REST API
- Update event validation to use dynamic event types
- Update slug generation to use custom slug_prefix

Frontend:
- Add EventTypesPage with full CRUD admin interface
- Add eventTypes.service.ts API client
- Update CreateEventPage to fetch types dynamically
- Add Event Types navigation in admin sidebar
- Add i18n translations (EN/DE)

Backward compatible: existing galleries continue to work, legacy types
accepted even if database is empty via fallback mechanisms.

frontend/src/App.tsx

@@ -24,7 +24,8 @@ import {
   SettingsPage,
   BackupManagement,
   CMSPage,
-  UserManagementPage
+  UserManagementPage,
+  EventTypesPage
 } from './pages/admin';
 import { AcceptInvitePage } from './pages/public/AcceptInvitePage';
 import { AdminLayout, AdminAuthWrapper } from './components/admin';
@@ -128,6 +129,7 @@ function App() {
                       <Route path="analytics" element={<AnalyticsPage />} />
                       <Route path="branding" element={<BrandingPage />} />
                       <Route path="settings" element={<SettingsPage />} />
+                      <Route path="event-types" element={<EventTypesPage />} />
                       <Route path="backup" element={<BackupManagement />} />
                       <Route path="cms" element={<CMSPage />} />
                       <Route path="users" element={<UserManagementPage />} />