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/config.py

@@ -221,6 +221,10 @@ class Settings(BaseSettings):
     plex_path_map: str = ""
     # Which Plex library section keys are exposed (comma-separated). Empty = all enabled sections.
     plex_libraries: str = ""
+    # HU-overlay mapping (E5 S5): comma-separated `huSectionKey:enSectionKey` pairs. Each named HU
+    # section is synced as a non-browsable overlay whose Hungarian metadata is written onto the EN
+    # twin section's rows (matched by guid). Empty = no overlays. E.g. "3:4,5:1".
+    plex_overlay_map: str = ""
     # Stable client identifier Siftlode presents to Plex (X-Plex-Client-Identifier).
     plex_client_id: str = "siftlode-server"
     # How often the catalog sync job mirrors Plex metadata, in minutes.