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/plex/client.py

@@ -85,6 +85,10 @@ class PlexClient:
             f"/library/sections/{key}/all",
             params={
                 "type": item_type,
+                # includeGuids=1 carries each item's Guid array (tmdb/imdb/tvdb ids) in the cheap
+                # listing, so the sync can capture the canonical guid without a per-item rich fetch —
+                # the pairing key for the HU overlay (E5 S5).
+                "includeGuids": 1,
                 "X-Plex-Container-Start": start,
                 "X-Plex-Container-Size": size,
             },