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.