feat(plex): collections — sync, browse, filter, and info-page strips (Phase 1, read)

Backend (migration 0047_plex_collections): a plex_collections table mirrors every Plex
collection (card metadata + smart flag + an editable flag reserved for Phase 2); membership
is stored as GIN-indexed collection_keys on member movies (plex_items) and shows (plex_shows).
The background sync (plex_sync) now fetches each library's collections + their children and
rebuilds membership — so ALL reads are local (Plex's dual-language collection queries are slow;
this trades a ~4-min background sync for instant reads). New /api/plex/collections endpoint;
/browse gains a combinable  filter; item_detail returns the movie's collection
'strips' (sibling titles as playable cards, smallest/most-specific collection first) — all pure
local lookups.

Frontend: PlexSidebar gains a searchable Collection picker + an active-collection chip;
PlexInfo renders the collection strips (playable posters + 'Browse collection' → sets the
filter); the collection is part of PlexFilters (persisted). i18n en/hu/de.

Phase 2 (create/edit collections with write-back to Plex) is separate.

backend/app/plex/client.py

@@ -95,6 +95,16 @@ class PlexClient:
         mc = self._get(f"/library/metadata/{rating_key}/children")
         return mc.get("Metadata", []) or []
 
+    def collections(self, section_key: str) -> list[dict]:
+        """All collections in a library section (title/summary/thumb/childCount/smart)."""
+        mc = self._get(f"/library/sections/{section_key}/collections")
+        return mc.get("Metadata", []) or []
+
+    def collection_children(self, rating_key: str) -> list[dict]:
+        """The member items (movies / shows) of a collection."""
+        mc = self._get(f"/library/collections/{rating_key}/children")
+        return mc.get("Metadata", []) or []
+
     def metadata(self, rating_key: str, markers: bool = False) -> dict | None:
         """Full metadata for one item; include intro/credit markers when requested."""
         params = {"includeMarkers": 1} if markers else None