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/plex/rollup.py

@@ -76,3 +76,21 @@ def state_map(db: Session, user_id: int, show_ids: list[int]) -> dict[int, str]:
         agg_subquery(db, user_id).where(PlexItem.show_id.in_(show_ids))
     ).all()
     return {r.show_id: show_status(r.total, r.watched, r.inprog) for r in rows}
+
+
+def item_state_map(
+    db: Session, user_id: int, item_ids: list[int]
+) -> dict[int, PlexState]:
+    """A user's PlexState rows for a set of leaf items (movies or episodes), keyed by item_id — the
+    leaf analogue of `state_map` (which rolls a show's episodes up to a status string). One batched
+    fetch, so the caller serializes cards without a per-card query. Empty in → empty dict (no `IN ()`)."""
+    if not item_ids:
+        return {}
+    return {
+        s.item_id: s
+        for s in db.execute(
+            select(PlexState).where(
+                PlexState.user_id == user_id, PlexState.item_id.in_(item_ids)
+            )
+        ).scalars()
+    }