Add collection/franchise ingestion

Collections were bare stubs (name only) lazily created the moment a
movie's belongs_to_collection reference was seen. Adds a real
ingest_collection pipeline: TMDb's /collection/{id} for overview and
poster/backdrop artwork, plus ordered CollectionMovie membership --
linking only movies already ingested into Aurora, the same
never-fabricate-a-stub rule movie recommendations already follow.

get_or_create_collection now refreshes name/overview on an existing
stub instead of only creating, guarded so a bare movie-stub call
(overview=None) never clobbers real data a full ingestion already
filled in.

New POST /api/v1/admin/ingest/collection endpoint and
ingest_collection_task; GET /api/v1/collections/{id} needed no changes
since its read path (collection_service.py) already assembled the full
detail shape.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

app/workers/tasks.py

@@ -28,6 +28,7 @@ from app.services.ingestion.runner import (
     get_series_company_ids,
     get_series_season_ids,
     get_series_season_numbers,
+    run_collection_ingestion,
     run_movie_ingestion,
     run_person_ingestion,
     run_season_ingestion,
@@ -102,6 +103,12 @@ async def ingest_season_task(
     return str(season_id)
 
 
+async def ingest_collection_task(ctx: dict[str, Any], job_id: str, external_id: str) -> str:
+    collection_id = await run_collection_ingestion(uuid.UUID(job_id), external_id)
+    await _cascade_artwork_downloads(ctx, "collection", [collection_id])
+    return str(collection_id)
+
+
 async def download_artwork_task(ctx: dict[str, Any], artwork_id: str) -> bool:
     async with session_scope() as session:
         return await download_artwork(session, get_artwork_storage(), uuid.UUID(artwork_id))