feat(plex): S5a HU-overlay backend — schema, guid, overlay sync

Adds the storage + sync for the Hungarian-metadata overlay (the browsable EN sections gain HU
twin fields written from a paired HU section, matched by guid; no browsable HU rows).

Schema (2 migrations, ADD-only; run before the first HU sync while *_hu is NULL so the generated
column build is trivial):
- 0063: plex_libraries.metadata_lang ('en' default) + self-FK overlay_of.
- 0064: plex_items/plex_shows.guid (indexed) + title_hu/summary_hu/genres_hu (+ GIN) + a STORED
  generated search_vector_hu mirroring the EN vector (title_hu A / people_text B / summary_hu C) + GIN.

Sync:
- includeGuids=1 on section_items so the canonical guid rides the cheap listing.
- _canonical_guid(meta) (tmdb > imdb > tvdb) captured onto every EN row in _apply_item/_sync_shows.
- Two-pass sync(): pass 1 mirrors browsable (non-overlay) sections; pass 2 reads plex_overlay_map
  ('huKey:enKey', new sysconfig), designates each HU section (metadata_lang='hu' + overlay_of), and
  _sync_overlay writes title_hu/summary_hu/genres_hu onto the guid-matched EN rows.

Browse: _content_libs now excludes overlay libraries (overlay_of is None); the unified_library and
facets inline library partitions fold into it, and list_libraries/collections exclude overlays too.

Tests: DB-lane coverage of the overlay write (matched vs unmatched rows, no browsable HU rows, the
generated search_vector_hu populates), the guid parse, and the browse exclusion. Full suite 272
passed; migrations verified reversible (upgrade→downgrade→upgrade). S5b (persist the live detail
fields + serve item/show detail from DB) is the follow-up.

backend/app/models.py

@@ -995,6 +995,13 @@ class PlexLibrary(Base, TimestampMixin, UpdatedAtMixin):
     kind: Mapped[str] = mapped_column(String(16))  # "movie" | "show"
     enabled: Mapped[bool] = mapped_column(Boolean, default=True, server_default="true")
     synced_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    # HU-overlay support (E5 S5). `metadata_lang` tags the section's metadata language. An OVERLAY
+    # library (overlay_of set) is the Hungarian-metadata twin of another section: it is never browsable
+    # on its own — its titles' HU fields are written onto the matched EN rows (paired by `guid`).
+    metadata_lang: Mapped[str] = mapped_column(String(8), default="en", server_default="en")
+    overlay_of: Mapped[int | None] = mapped_column(
+        ForeignKey("plex_libraries.id", ondelete="SET NULL"), index=True
+    )
 
 
 class PlexShow(Base, TimestampMixin, UpdatedAtMixin):
@@ -1007,6 +1014,7 @@ class PlexShow(Base, TimestampMixin, UpdatedAtMixin):
         Index("ix_plex_shows_genres_gin", "genres", postgresql_using="gin"),
         Index("ix_plex_shows_directors_gin", "directors", postgresql_using="gin"),
         Index("ix_plex_shows_cast_gin", "cast_names", postgresql_using="gin"),
+        Index("ix_plex_shows_genres_hu_gin", "genres_hu", postgresql_using="gin"),
         # ix_plex_shows_collections_gin already created with collections (migration 0047).
     )
 
@@ -1015,6 +1023,9 @@ class PlexShow(Base, TimestampMixin, UpdatedAtMixin):
     library_id: Mapped[int] = mapped_column(
         ForeignKey("plex_libraries.id", ondelete="CASCADE"), index=True
     )
+    # Canonical external id (e.g. "tmdb:12345") — the key that pairs this EN row with its HU-overlay
+    # twin during sync (E5 S5). Indexed for the guid->row overlay match.
+    guid: Mapped[str | None] = mapped_column(String(64), index=True)
     title: Mapped[str] = mapped_column(String(512))
     summary: Mapped[str | None] = mapped_column(Text)
     year: Mapped[int | None] = mapped_column(Integer)
@@ -1037,6 +1048,11 @@ 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))
+    # 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))
+    summary_hu: Mapped[str | None] = mapped_column(Text)
+    genres_hu: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
     search_vector: Mapped[object | None] = mapped_column(
         TSVECTOR,
         Computed(
@@ -1046,6 +1062,15 @@ class PlexShow(Base, TimestampMixin, UpdatedAtMixin):
             persisted=True,
         ),
     )
+    search_vector_hu: Mapped[object | None] = mapped_column(
+        TSVECTOR,
+        Computed(
+            "setweight(to_tsvector('public.unaccent_simple', coalesce(title_hu, '')), 'A') || "
+            "setweight(to_tsvector('public.unaccent_simple', coalesce(people_text, '')), 'B') || "
+            "setweight(to_tsvector('public.unaccent_simple', left(coalesce(summary_hu, ''), 1000)), 'C')",
+            persisted=True,
+        ),
+    )
 
 
 class PlexSeason(Base, TimestampMixin, UpdatedAtMixin):
@@ -1077,6 +1102,7 @@ class PlexItem(Base, TimestampMixin, UpdatedAtMixin):
         Index("ix_plex_items_directors_gin", "directors", postgresql_using="gin"),
         Index("ix_plex_items_cast_gin", "cast_names", postgresql_using="gin"),
         Index("ix_plex_items_collections_gin", "collection_keys", postgresql_using="gin"),
+        Index("ix_plex_items_genres_hu_gin", "genres_hu", postgresql_using="gin"),
     )
 
     id: Mapped[int] = mapped_column(primary_key=True)
@@ -1085,6 +1111,9 @@ class PlexItem(Base, TimestampMixin, UpdatedAtMixin):
         ForeignKey("plex_libraries.id", ondelete="CASCADE"), index=True
     )
     kind: Mapped[str] = mapped_column(String(16), index=True)  # "movie" | "episode"
+    # Canonical external id (e.g. "tmdb:12345") — pairs a MOVIE row with its HU-overlay twin during
+    # sync (E5 S5); indexed for the guid->row match. Episodes carry it too but it's unused for them.
+    guid: Mapped[str | None] = mapped_column(String(64), index=True)
     show_id: Mapped[int | None] = mapped_column(
         ForeignKey("plex_shows.id", ondelete="CASCADE"), index=True
     )
@@ -1130,6 +1159,11 @@ 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))
+    # 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))
+    summary_hu: Mapped[str | None] = mapped_column(Text)
+    genres_hu: Mapped[list | None] = mapped_column(JSONB(none_as_null=True))
     search_vector: Mapped[object | None] = mapped_column(
         TSVECTOR,
         Computed(
@@ -1139,6 +1173,15 @@ class PlexItem(Base, TimestampMixin, UpdatedAtMixin):
             persisted=True,
         ),
     )
+    search_vector_hu: Mapped[object | None] = mapped_column(
+        TSVECTOR,
+        Computed(
+            "setweight(to_tsvector('public.unaccent_simple', coalesce(title_hu, '')), 'A') || "
+            "setweight(to_tsvector('public.unaccent_simple', coalesce(people_text, '')), 'B') || "
+            "setweight(to_tsvector('public.unaccent_simple', left(coalesce(summary_hu, ''), 1000)), 'C')",
+            persisted=True,
+        ),
+    )
 
 
 class PlexPerson(Base, TimestampMixin, UpdatedAtMixin):