fix(plex): never shrink cast_names on sync so a watermark reset can't regress it (B-sub-2)

_apply_facet_fields seeded cast_names from the cheap top-~20 listing only while
people_enriched_at was NULL, relying on "a sync never clears the watermark" to
protect an enriched fuller cast. Migration 0065 broke that assumption by nulling
people_enriched_at on every movie/show row, so the first post-deploy sync
downgraded enriched casts back to the listing top-20 — silently dropping
person-page cast-filter names for weeks until the backfill caught up (the S5b
regression, re-opened by the migration).

Guard structurally instead of trusting the watermark: union the listing cast
onto whatever is stored, never dropping a name already present. A fresh row
still seeds from the listing; a row whose watermark was reset but whose fuller
cast survives keeps it. Robust to any future watermark reset, not just 0065.

backend/app/plex/sync.py

@@ -305,11 +305,17 @@ def _apply_facet_fields(row, meta: dict) -> None:
     row.genres = _tags(meta, "Genre")
     row.directors = _tags(meta, "Director")
     # The listing carries only the top ~20 cast, so re-applying it would DOWNGRADE an already-enriched
-    # full cast (up to 24, grown by the rich backfill/info-page enrich) — and since a sync never clears
-    # `people_enriched_at`, nothing would regrow it, silently breaking the person-page cast filter for
-    # names past the top ~20. Only take the listing's cast before the row is enriched; keep it after.
+    # full cast (up to 24, grown by the rich backfill/info-page enrich), silently breaking the
+    # person-page cast filter for names past the top ~20. The watermark gate below (`people_enriched_at
+    # is None`) normally protects this, but it is NOT enough on its own: a migration that resets the
+    # watermark re-opens the regression (0065 nulled it, which is exactly how the S5b fix got undone in
+    # prod). So guard STRUCTURALLY too — union the listing cast onto whatever is stored, never dropping a
+    # name we already have. On a fresh row (empty `cast_names`) this seeds the top ~20; on a row whose
+    # watermark was reset but whose fuller cast survives, it keeps the fuller cast. Robust to any future
+    # watermark reset, not just this one.
     if row.people_enriched_at is None:
-        row.cast_names = _tags(meta, "Role", limit=20)
+        merged = list(dict.fromkeys((row.cast_names or []) + (_tags(meta, "Role", limit=20) or [])))
+        row.cast_names = merged or None
     people = list(dict.fromkeys((row.directors or []) + (row.cast_names or [])))
     row.people_text = " ".join(people) or None