Lazy enrichment only covers viewed titles, so an actor's count/results were
partial. Add a background backfill: each plex_sync rich-fetches a bounded batch
(60) of not-yet-enriched movies/shows and persists their FULL cast, stamping
people_enriched_at so each is fetched once — the whole library is covered over
successive runs (migration 0057 adds the marker). Extracted the enrichment into a
shared app/plex/people.py (cast_from_meta / enrich_row_people / prune_orphan_people
+ the person-image host/proxy) so the info-page lazy path and the batch share ONE
implementation — no duplication; the info page also stamps the marker so the batch
skips it. Verified: an 8-item batch grows those movies' cast_names 3->24 and fills
plex_people (32->224); the info-page cast is intact after the _rich_meta refactor.
(Plex cast-explore S1b)
