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/api/v1/admin.py

@@ -49,6 +49,10 @@ class IngestSeasonRequest(BaseModel):
     season_number: int
 
 
+class IngestCollectionRequest(BaseModel):
+    tmdb_id: str
+
+
 class IngestionEnqueuedOut(BaseModel):
     job_id: str
     arq_job_id: str
@@ -146,6 +150,24 @@ async def enqueue_season_ingestion(
     )
 
 
+@router.post("/ingest/collection", response_model=IngestionEnqueuedOut)
+async def enqueue_collection_ingestion(
+    body: IngestCollectionRequest, queue: ArqRedis = Depends(get_queue)
+) -> IngestionEnqueuedOut:
+    # Membership only links movies already ingested into Aurora (never a
+    # stub) -- see app/services/ingestion/collection_ingestion.py -- so
+    # ingesting the franchise's movies first (or re-running this afterward)
+    # is what fills the list in.
+    return await _enqueue(
+        queue,
+        job_type="collection_sync",
+        entity_type="collection",
+        external_id=body.tmdb_id,
+        task_name="ingest_collection_task",
+        task_args=(body.tmdb_id,),
+    )
+
+
 @router.get("/ingest/jobs/{job_id}", response_model=IngestionJobOut)
 async def get_ingestion_job_status(
     job_id: uuid.UUID, session: AsyncSession = Depends(get_db)