feat(plex): S5b persist rich detail + serve item/show detail from DB

Persist the info-page rich fields that were previously read LIVE from plex.metadata() on every view —
imdb_rating (IMDb-specific, vs `rating`=audienceRating), imdb_id, tagline, and full_cast
([{name,role,thumb}]) — onto plex_items/plex_shows, and serve the detail endpoints from the row.

- migration 0065: ADD imdb_rating/imdb_id/tagline/full_cast (nullable → no rewrite/lock, unlike 0064);
  NULL people_enriched_at on movie/show rows so the existing cast backfill re-fetches each once and
  fills the new columns.
- new app/plex/detail.py: rich_detail(meta) (moved out of browse._rich_meta so sync + serve share one
  parser) + apply_rich_detail(row, meta); wired into _enrich_people_batch and the info-page lazy fill.
- item_detail: movies serve cast/IMDb/tagline/genres from the row and keep plex.metadata(markers=True)
  ONLY for markers + audio/subtitle streams (so detail survives a live-Plex outage); episodes (never
  enriched) still parse detail from that live meta.
- show_detail: serves cast/IMDb from the row; `related` stays live best-effort and also lazily fills a
  not-yet-enriched show.
- tests: rich_detail/apply_rich_detail parsing + persistence, backfill stamping, item_detail
  outage-resilience + lazy fill + episode-live, show_detail row serving. Full suite 285 pass / 2 skip;
  migration verified reversible (up→down→up).

Precondition for S6's pure-DB language-aware serving.

backend/app/plex/sync.py

@@ -17,7 +17,7 @@ from sqlalchemy.orm import Session
 
 from app import sysconfig
 from app.models import PlexCollection, PlexItem, PlexLibrary, PlexSeason, PlexShow
-from app.plex import people
+from app.plex import detail, people
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
 from app.plex.lookups import by_rating_key
 
@@ -258,6 +258,7 @@ def _enrich_people_batch(db: Session, plex: PlexClient, limit: int = _ENRICH_BAT
             log.debug("Plex people enrich fetch failed for %s: %s", row.rating_key, e)
             continue  # leave people_enriched_at NULL so it's retried on a later run
         people.enrich_row_people(db, row, people.cast_from_meta(meta))
+        detail.apply_rich_detail(row, meta)  # imdb/tagline/full_cast → served from the row, not live
         row.people_enriched_at = now
         n += 1
     db.commit()