Add series/season/episode ingestion pipeline

Movie and person ingestion existed, but the series/season/episode API
endpoints had no way to actually populate them from TMDb -- the
biggest remaining gap in Core 0.1's television scope.

- normalize_series/normalize_season in the metadata normalizer,
  including two TMDb quirks that would otherwise silently produce
  empty data: /tv nests keywords and alternative_titles under
  "results" (not "keywords"/"titles" like /movie), and series-level
  creators come from a separate created_by array, not the credits
  cast/crew lists.
- ingest_series upserts the Series row plus season *stubs* (number,
  title, air date, poster -- everything TMDb inlines on /tv/{id});
  ingest_season fetches and upserts one season's full episode list,
  and requires the series to already exist (TMDb has no single
  endpoint for a whole series' episodes, so this has to be a separate
  per-season call).
- Wired through the same job-lifecycle path as movie/person ingestion
  (runner.py, arq tasks, admin endpoints), with the shared boilerplate
  between them factored into runner._run_ingestion and
  admin.py:_enqueue rather than copy-pasted a third and fourth time.
- Unit tests for both normalizers (runnable without a DB) and
  integration tests for the full ingest_series -> ingest_season flow,
  including the series-must-exist-first error and idempotency on
  re-ingestion.

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

docs/api.md

@@ -120,6 +120,8 @@ you want network-level isolation too.
 |---|---|---|---|
 | POST | `/api/v1/admin/ingest/movie` | `{"tmdb_id": "603"}` | Enqueues an arq job; returns `{"job_id": "...", "status": "enqueued"}` immediately (section 15: ingestion never runs inline in a request). |
 | POST | `/api/v1/admin/ingest/person` | `{"tmdb_id": "6384"}` | Same, for a person profile. |
+| POST | `/api/v1/admin/ingest/series` | `{"tmdb_id": "1396"}` | Ingests the series and season *stubs* (number, title, air date, poster) — no episodes yet. |
+| POST | `/api/v1/admin/ingest/season` | `{"tmdb_id": "1396", "season_number": 1}` | Ingests one season's full episode list. Requires the series to already be ingested (`ingest/series` first) — fails with a clear error otherwise. |
 | GET | `/api/v1/admin/ingest/jobs/{job_id}` | — | Poll job status: `pending` → `running` → `succeeded`/`failed`/`retrying`. |
 
 ## Errors