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/models.py

@@ -1048,6 +1048,12 @@ class PlexShow(Base, TimestampMixin, UpdatedAtMixin):
     people_text: Mapped[str | None] = mapped_column(Text)  # cast+directors, folded into search_vector
     # Set once the show's FULL cast has been rich-fetched by the backfill; NULL = still the listing top-3.
     people_enriched_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    # Rich-fetch-only detail fields, persisted by the same backfill that fills the full cast (E5 S5b) so
+    # the info page serves from this row instead of a live plex.metadata() call. NULL until enriched.
+    imdb_rating: Mapped[float | None] = mapped_column(Float)  # IMDb-specific score (vs `rating` = audienceRating)
+    imdb_id: Mapped[str | None] = mapped_column(String(16))  # tt… → the clickable IMDb link
+    tagline: Mapped[str | None] = mapped_column(Text)
+    full_cast: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))  # ordered [{name, role, thumb}]
     # Hungarian-metadata twin fields, written by the overlay sync (E5 S5). NULL = no HU twin matched,
     # so serving falls back to the EN columns. `people_text` is language-neutral, reused for both vectors.
     title_hu: Mapped[str | None] = mapped_column(String(512))
@@ -1159,6 +1165,12 @@ class PlexItem(Base, TimestampMixin, UpdatedAtMixin):
     people_text: Mapped[str | None] = mapped_column(Text)
     # Set once the movie's FULL cast has been rich-fetched by the backfill; NULL = still the listing top-3.
     people_enriched_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    # Rich-fetch-only detail fields, persisted by the same backfill that fills the full cast (E5 S5b) so
+    # the info page serves from this row instead of a live plex.metadata() call. NULL until enriched.
+    imdb_rating: Mapped[float | None] = mapped_column(Float)  # IMDb-specific score (vs `rating` = audienceRating)
+    imdb_id: Mapped[str | None] = mapped_column(String(16))  # tt… → the clickable IMDb link
+    tagline: Mapped[str | None] = mapped_column(Text)
+    full_cast: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))  # ordered [{name, role, thumb}]
     # Hungarian-metadata twin fields, written by the overlay sync (E5 S5). NULL = no HU twin matched
     # (serving falls back to the EN columns). `people_text` is language-neutral, reused for both vectors.
     title_hu: Mapped[str | None] = mapped_column(String(512))