feat(categories): per-event category ordering — global default + override (#782)

Order a gallery's categories in the flow of the day instead of A–Z. Two layers,
resolved per event: per-event override > global default > name.

- migration 158: photo_categories.display_order (global default), backfilled
  from the current alphabetical order so existing galleries don't reshuffle.
- migration 159: event_category_order (event_id, category_id, position) — the
  per-event override; no backfill, every event starts on the default.
- utils/categoryOrder: shared resolution used by the admin event view and the
  public gallery; fails safe to the global default if the table is absent.
- adminCategories: POST /reorder sets a per-event override (globals +
  event-specific, interleaved); DELETE /reorder/:eventId resets; POST
  /reorder-global sets the global default. Ordering endpoints + create append.
- gallery renders the resolved order.
- Settings → Photo Categories reorders the global default; an event's Categories
  tab reorders that gallery (one combined list + Reset to default). Up/down
  buttons — no drag-and-drop dependency.
- en/de strings.

frontend/src/services/categories.service.ts

@@ -10,6 +10,12 @@ export interface PhotoCategory {
   // Per-category download permission (#640). Defaults true (server-side) so
   // categories created before migration 135 keep working.
   allow_downloads?: boolean;
+  // Global default sort order (#782). Backfilled from the previous alphabetical
+  // order on migration, so existing galleries don't reshuffle.
+  display_order?: number;
+  // Per-event override position (#782). Non-null on the /event/:id response when
+  // this gallery has customised its order; null means it follows the default.
+  override_position?: number | null;
   created_at: string;
 }
 
@@ -61,5 +67,31 @@ export const categoriesService = {
   // Delete a category
   async deleteCategory(id: number): Promise<void> {
     await api.delete(`/admin/categories/${id}`);
+  },
+
+  // Set a per-event order override (#782). Sends the full ordered id list for
+  // this event — globals + event-specific — and returns the resolved order.
+  // Overrides the global default for this gallery only.
+  async reorderCategories(eventId: number, orderedIds: number[]): Promise<PhotoCategory[]> {
+    const response = await api.post<PhotoCategory[]>('/admin/categories/reorder', {
+      event_id: eventId,
+      orderedIds
+    });
+    return response.data;
+  },
+
+  // Clear an event's override — revert this gallery to the global default order.
+  async resetEventOrder(eventId: number): Promise<PhotoCategory[]> {
+    const response = await api.delete<PhotoCategory[]>(`/admin/categories/reorder/${eventId}`);
+    return response.data;
+  },
+
+  // Set the GLOBAL default order for shared categories (#782). Applies to every
+  // gallery that hasn't set its own override.
+  async reorderGlobalCategories(orderedIds: number[]): Promise<PhotoCategory[]> {
+    const response = await api.post<PhotoCategory[]>('/admin/categories/reorder-global', {
+      orderedIds
+    });
+    return response.data;
   }
 };