refactor(e5): deferred review-tail (backend) — enrich off request path, reuse, tests

Address the deep-review's deferred backend items so E5 is review-complete:

- person-enrich OFF the request path (ABUSE finding): person_detail no longer makes
  the TMDB call inline (even bounded to 8s it sat on the serve thread, un-throttled,
  demo-reachable — it violated the module's "no live TMDB call at serve time"
  invariant). It now schedules a FastAPI BackgroundTask (people._enrich_person_bg,
  fresh session, best-effort) after the response; the first open serves the row as-is
  and the bio lands by the next open. The inline try/rollback recovery is gone with it.

- reuse: a shared tmdb.resolve_library_guids owns the collision-safe guid→library
  bucketing (movie vs tv ids are independent sequences) that resolve_recs and
  people._resolve_known_for each hand-duplicated; a new rollup.item_state_map dedups
  the "PlexState-by-item_id" fetch across the person + discover paths.

- test gaps: enrich_batch transient TmdbError leaves the watermark NULL to retry;
  guid_to_tmdb_id resolves a tvdb source + tv media_type via /find's tv_results;
  _continue_watching includes a NULL-status started movie and excludes a hidden one;
  /api/search/youtube returns 422 (not 5xx) when ingest fails (route-level, B-core-1).

Backend lane 403 → 413, ruff clean. No migration; serve contracts unchanged.

backend/tests/test_plex_discover.py

@@ -113,6 +113,33 @@ def test_continue_watching_movies_and_shows(db, monkeypatch):
     assert "m1" not in ids
 
 
+def test_continue_watching_null_status_in_hidden_out(db, monkeypatch):
+    # A started movie whose PlexState.status is NULL is still "in progress": `NULL NOT IN (...)` is NULL
+    # (not TRUE), so a bare notin_ would wrongly drop it — the or_(is_(None), notin_) branch keeps it.
+    # A hidden started movie must be excluded.
+    lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
+    db.add(lib)
+    db.flush()
+    m_null = PlexItem(rating_key="mn", library_id=lib.id, kind="movie", title="Null Status", guid="tmdb:1")
+    m_hidden = PlexItem(rating_key="mh", library_id=lib.id, kind="movie", title="Hidden", guid="tmdb:2")
+    db.add_all([m_null, m_hidden])
+    db.flush()
+    user = User(email="cw-null@example.com")
+    db.add(user)
+    db.flush()
+    db.add_all([
+        PlexState(user_id=user.id, item_id=m_null.id, status=None, position_seconds=500,
+                  progress_updated_at=NOW),
+        PlexState(user_id=user.id, item_id=m_hidden.id, status="hidden", position_seconds=500,
+                  progress_updated_at=NOW - timedelta(hours=1)),
+    ])
+    db.commit()
+
+    ids = _ids(_get(db, user, monkeypatch, "/api/plex/discover?scope=movie").json()["continue_watching"])
+    assert "mn" in ids  # NULL-status started movie IS in progress
+    assert "mh" not in ids  # hidden is excluded
+
+
 def test_because_you_watched_resolves_in_library_recs(db, monkeypatch):
     user = _seed(db)
     d = _get(db, user, monkeypatch, "/api/plex/discover?scope=both").json()