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/routes/plex/browse.py

@@ -82,9 +82,12 @@ def _field_recheck(model, field: str, tsq):
 def list_libraries(
     user: User = Depends(current_user), db: Session = Depends(get_db)
 ) -> dict:
-    """The mirrored libraries the user can browse (drives the Plex scope selector)."""
+    """The mirrored libraries the user can browse (drives the Plex scope selector). HU overlay
+    libraries are excluded — they're not browsable, they only feed the EN twin's Hungarian metadata."""
     libs = db.execute(
-        select(PlexLibrary).order_by(PlexLibrary.kind.desc(), PlexLibrary.title)
+        select(PlexLibrary)
+        .where(PlexLibrary.overlay_of.is_(None))
+        .order_by(PlexLibrary.kind.desc(), PlexLibrary.title)
     ).scalars().all()
     out = []
     for lib in libs:
@@ -291,11 +294,7 @@ def unified_library(
     episodes come back in a separate `episodes` list (grouped result — the 'Episodes' section)."""
     offset = max(0, offset)
     p = f.as_dict()
-    libs = db.execute(
-        select(PlexLibrary).where(PlexLibrary.enabled.is_(True))
-    ).scalars().all()
-    movie_lib_ids = [lb.id for lb in libs if lb.kind == "movie"]
-    show_lib_ids = [lb.id for lb in libs if lb.kind == "show"]
+    movie_lib_ids, show_lib_ids = _content_libs(db)
     want_movies = scope in ("movie", "both") and bool(movie_lib_ids)
     want_shows = scope in ("show", "both") and bool(show_lib_ids)
 
@@ -531,17 +530,9 @@ def facets(
         "duration_min": None,
         "duration_max": None,
     }
-    libs = db.execute(
-        select(PlexLibrary).where(PlexLibrary.enabled.is_(True))
-    ).scalars().all()
-    movie_ids = (
-        [lb.id for lb in libs if lb.kind == "movie"]
-        if scope in ("movie", "both")
-        else []
-    )
-    show_ids = (
-        [lb.id for lb in libs if lb.kind == "show"] if scope in ("show", "both") else []
-    )
+    all_movie_ids, all_show_ids = _content_libs(db)
+    movie_ids = all_movie_ids if scope in ("movie", "both") else []
+    show_ids = all_show_ids if scope in ("show", "both") else []
     if not movie_ids and not show_ids:
         return empty
     p = f.as_dict()
@@ -758,7 +749,9 @@ def collections(
     else:
         enabled_ids = list(
             db.execute(
-                select(PlexLibrary.id).where(PlexLibrary.enabled.is_(True))
+                select(PlexLibrary.id).where(
+                    PlexLibrary.enabled.is_(True), PlexLibrary.overlay_of.is_(None)
+                )
             ).scalars()
         )
         if not enabled_ids:
@@ -1545,11 +1538,13 @@ def _rich_meta(meta: dict) -> dict:
 
 
 def _content_libs(db: Session) -> tuple[list[int], list[int]]:
-    """The enabled content libraries, partitioned into (movie_lib_ids, show_lib_ids) — the single
-    source of the browsable-library scope shared by people_cards and the person filmography endpoint.
-    (S5 will extend this to exclude HU overlay libraries and fold in the browse/facets partition sites.)"""
+    """The BROWSABLE content libraries, partitioned into (movie_lib_ids, show_lib_ids) — the single
+    source of the browse scope. Excludes HU overlay libraries (`overlay_of` set): those are never
+    browsed on their own, they only lend their Hungarian metadata to the EN twin's rows (E5 S5)."""
     libs = db.execute(
-        select(PlexLibrary).where(PlexLibrary.enabled.is_(True))
+        select(PlexLibrary).where(
+            PlexLibrary.enabled.is_(True), PlexLibrary.overlay_of.is_(None)
+        )
     ).scalars().all()
     movie_lib_ids = [lb.id for lb in libs if lb.kind == "movie"]
     show_lib_ids = [lb.id for lb in libs if lb.kind == "show"]