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.

backend/server.js

@@ -437,6 +437,7 @@ app.use('/api/admin/photo-export', require('./src/routes/adminPhotoExport'));
 app.use('/api/admin/css-templates', require('./src/routes/adminCssTemplates'));
 app.use('/api/admin/events', require('./src/routes/adminEventRename'));
 app.use('/api/admin/users', require('./src/routes/adminUsers'));
+app.use('/api/admin/event-types', require('./src/routes/adminEventTypes'));
 app.use('/api/invite', require('./src/routes/acceptInvite'));
 app.use('/api/public/settings', require('./src/routes/publicSettings'));
 app.use('/api/public', require('./src/routes/publicCMS'));