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/services/eventTypes.service.ts

@@ -0,0 +1,100 @@
+/**
+ * Event Types Service
+ * API client for event type management
+ */
+
+import { api } from '../config/api';
+
+export interface EventType {
+  id: number;
+  name: string;
+  slug_prefix: string;
+  emoji: string;
+  theme_preset: string;
+  theme_config: string | null;
+  display_order: number;
+  is_system: boolean;
+  is_active: boolean;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface CreateEventTypeData {
+  name: string;
+  slug_prefix: string;
+  emoji?: string;
+  theme_preset?: string;
+  theme_config?: Record<string, unknown>;
+  display_order?: number;
+}
+
+export interface UpdateEventTypeData {
+  name?: string;
+  slug_prefix?: string;
+  emoji?: string;
+  theme_preset?: string;
+  theme_config?: Record<string, unknown>;
+  display_order?: number;
+  is_active?: boolean;
+}
+
+export const eventTypesService = {
+  /**
+   * Get all event types (for admin management)
+   */
+  async getEventTypes(includeInactive = false): Promise<EventType[]> {
+    const response = await api.get<{ eventTypes: EventType[] }>('/admin/event-types', {
+      params: { includeInactive: includeInactive.toString() }
+    });
+    return response.data.eventTypes;
+  },
+
+  /**
+   * Get only active event types (for dropdowns/selection)
+   */
+  async getActiveEventTypes(): Promise<EventType[]> {
+    const response = await api.get<{ eventTypes: EventType[] }>('/admin/event-types/active');
+    return response.data.eventTypes;
+  },
+
+  /**
+   * Get a single event type by ID
+   */
+  async getEventType(id: number): Promise<EventType> {
+    const response = await api.get<EventType>(`/admin/event-types/${id}`);
+    return response.data;
+  },
+
+  /**
+   * Create a new event type
+   */
+  async createEventType(data: CreateEventTypeData): Promise<EventType> {
+    const response = await api.post<EventType>('/admin/event-types', data);
+    return response.data;
+  },
+
+  /**
+   * Update an event type
+   */
+  async updateEventType(id: number, data: UpdateEventTypeData): Promise<EventType> {
+    const response = await api.put<EventType>(`/admin/event-types/${id}`, data);
+    return response.data;
+  },
+
+  /**
+   * Delete an event type
+   */
+  async deleteEventType(id: number): Promise<void> {
+    await api.delete(`/admin/event-types/${id}`);
+  },
+
+  /**
+   * Reorder event types
+   */
+  async reorderEventTypes(orderedIds: number[]): Promise<EventType[]> {
+    const response = await api.post<{ eventTypes: EventType[] }>('/admin/event-types/reorder', {
+      orderedIds
+    });
+    return response.data.eventTypes;
+  }
+};