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/app/routes/plex/discover.py

@@ -37,19 +37,8 @@ def _scope_libs(db: Session, scope: str) -> tuple[list[int], list[int]]:
     )
 
 
-def _movie_states(db: Session, uid: int, items: list[PlexItem]) -> dict[int, PlexState]:
-    """One batched fetch of the user's watch-state for a set of movies (avoids a per-card query)."""
-    ids = [it.id for it in items]
-    if not ids:
-        return {}
-    rows = db.execute(
-        select(PlexState).where(PlexState.user_id == uid, PlexState.item_id.in_(ids))
-    ).scalars().all()
-    return {r.item_id: r for r in rows}
-
-
 def _movie_cards(db: Session, uid: int, items: list[PlexItem]) -> list[dict]:
-    sts = _movie_states(db, uid, items)
+    sts = rollup.item_state_map(db, uid, [it.id for it in items])
     return [movie_card(it, sts.get(it.id)) for it in items]