feat(api): playlist add-items + search all-items endpoints (#208 #209)

- POST /api/playlists/{id:int}/items wraps the existing AddItemsToPlaylist
  command (mirrors CollectionController.AddItems); controller pre-checks
  playlist existence for a real 404, and the handler now rejects adds to
  system (generated) playlists, matching the guard already applied to
  rename/delete/replace-items so the Blazor path gets the same protection.
- GET /api/search/all-items wraps the existing QuerySearchIndexAllItems
  query, returning a new SearchResultAllItemsResponseModel (never expose
  the VM directly) so the SPA's shared "add all to collection/playlist"
  component can materialize ids before calling the add endpoints, same
  two-step flow Blazor's Search.razor already uses.
- Show-detail DTO check: ShowDetailResponseModel already exposes
  libraryId, title, and mediaSourceKind (serialized as a string enum via
  the global StringEnumConverter) - no changes needed.

Adds controller tests (route table + per-action) for both endpoints and
regenerates the OpenAPI document, endpoint index, and SPA client types.

web/src/api/generated/v1.d.ts

@@ -20,6 +20,18 @@ export interface components {
     "songIds": null | Array<number>;
     "imageIds": null | Array<number>;
     "remoteStreamIds": null | Array<number>;
+  };
+    "AddItemsToPlaylistRequest": {
+    "movieIds": null | Array<number>;
+    "showIds": null | Array<number>;
+    "seasonIds": null | Array<number>;
+    "episodeIds": null | Array<number>;
+    "artistIds": null | Array<number>;
+    "musicVideoIds": null | Array<number>;
+    "otherVideoIds": null | Array<number>;
+    "songIds": null | Array<number>;
+    "imageIds": null | Array<number>;
+    "remoteStreamIds": null | Array<number>;
   };
     "AddTraktListRequest": {
     "url": null | string;
@@ -1313,6 +1325,18 @@ export interface components {
     "SchedulingPickerOptionResponseModel": {
     "id": number;
     "name": string;
+  };
+    "SearchResultAllItemsResponseModel": {
+    "movieIds": null | Array<number>;
+    "showIds": null | Array<number>;
+    "seasonIds": null | Array<number>;
+    "episodeIds": null | Array<number>;
+    "artistIds": null | Array<number>;
+    "musicVideoIds": null | Array<number>;
+    "otherVideoIds": null | Array<number>;
+    "songIds": null | Array<number>;
+    "imageIds": null | Array<number>;
+    "remoteStreamIds": null | Array<number>;
   };
     "SearchResultGroupResponseModel": {
     "totalCount": number;