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/database.md

@@ -163,3 +163,14 @@ indexes on the alternate-title tables. See
   links) are replaced wholesale on each re-sync rather than diffed, so their
   Aurora IDs are not stable across re-ingestion — see
   `app/services/ingestion/movie_ingestion.py`.
+- Seasons don't get their own `external_ids` row (identified instead by the
+  natural `(series_id, season_number)` key) — episodes do, since TMDb gives
+  each episode a distinct global id. Revisit if a provider ever needs a
+  season-level external reference Aurora doesn't already have another way
+  to resolve.
+- `ingest_series` only upserts season *stubs* (number, title, air date,
+  poster) — episodes require a separate `ingest_season` call per season
+  (TMDb has no single endpoint returning a whole series' episodes), and
+  that call requires the series to already exist. There's no orchestration
+  yet that walks all of a series' seasons automatically after
+  `ingest_series` succeeds.