fix(plex): stop /facets 500 on a metadata-poor library

A mirrored Shows library whose items have no genres stored `genres` as JSON
`null` — SQLAlchemy's JSONB persists a Python None as a JSON scalar, not SQL
NULL. /facets unnested it with jsonb_array_elements_text(), which raises
"cannot extract elements from a scalar", so a single bare library 500'd the
whole Plex module on open (the grid loads, the filter sidebar's facets call dies).

- Guard the /facets genre unnest with jsonb_typeof(genres)='array', mirroring
  the guard people.py already applies to the same columns.
- Persist the JSONB tag columns (genres/directors/cast_names/collection_keys on
  plex_items and plex_shows) with none_as_null=True, so an empty list round-trips
  as SQL NULL and self-heals on the next sync — closing the trap for any future
  unnest.

Verified against the real prod row (DumaTV show, genres = JSON null): the
unguarded query errors, the guarded one returns cleanly.

backend/app/models.py

@@ -1020,16 +1020,19 @@ class PlexShow(Base, TimestampMixin, UpdatedAtMixin):
     thumb_key: Mapped[str | None] = mapped_column(String(512))  # Plex thumb path (proxied, not stored)
     art_key: Mapped[str | None] = mapped_column(String(512))
     child_count: Mapped[int | None] = mapped_column(Integer)  # seasons
-    collection_keys: Mapped[list | None] = mapped_column(JSONB)  # Plex collections this show is in
+    collection_keys: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))  # Plex collections this show is in
     added_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), index=True)
     # Filterable / orderable metadata mirrored from the show section listing (parallels PlexItem).
     rating: Mapped[float | None] = mapped_column(Float, index=True)  # Plex audienceRating (~IMDb)
     content_rating: Mapped[str | None] = mapped_column(String(16), index=True)
     studio: Mapped[str | None] = mapped_column(String(255), index=True)  # network/studio
     originally_available_at: Mapped[Date | None] = mapped_column(Date, index=True)  # first air date
-    genres: Mapped[list | None] = mapped_column(JSONB)
-    directors: Mapped[list | None] = mapped_column(JSONB)
-    cast_names: Mapped[list | None] = mapped_column(JSONB)
+    # none_as_null: an empty tag list must persist as SQL NULL, not JSON `null`. Without it a Python
+    # None round-trips as a JSON scalar that jsonb_array_elements_text() can't unnest ("cannot extract
+    # elements from a scalar") — the trap a metadata-poor library sprang on /facets. Same on plex_items.
+    genres: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
+    directors: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
+    cast_names: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
     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))
@@ -1116,11 +1119,11 @@ class PlexItem(Base, TimestampMixin, UpdatedAtMixin):
     content_rating: Mapped[str | None] = mapped_column(String(16), index=True)
     studio: Mapped[str | None] = mapped_column(String(255), index=True)
     originally_available_at: Mapped[Date | None] = mapped_column(Date, index=True)
-    genres: Mapped[list | None] = mapped_column(JSONB)
-    directors: Mapped[list | None] = mapped_column(JSONB)
-    cast_names: Mapped[list | None] = mapped_column(JSONB)
+    genres: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
+    directors: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
+    cast_names: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
     # rating_keys of the Plex collections this movie belongs to (GIN — filter + card-strip siblings).
-    collection_keys: Mapped[list | None] = mapped_column(JSONB)
+    collection_keys: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
     # Cast + director names, space-joined — folded into search_vector (weight B) so the search box
     # finds titles by a person's name (and ranks them above summary-only mentions).
     people_text: Mapped[str | None] = mapped_column(Text)