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.

frontend/src/i18n/locales/en/plex.json

@@ -33,7 +33,10 @@
     "addedOpt": { "24h": "24h", "1w": "1 week", "1m": "1 month", "6m": "6 months", "1y": "1 year" },
     "contentRating": "Age rating",
     "match": { "any": "Any", "all": "All" },
-    "dir": { "desc": "↓ Descending", "asc": "↑ Ascending" }
+    "dir": { "desc": "↓ Descending", "asc": "↑ Ascending" },
+    "collection": "Collection",
+    "collectionSearch": "Search collections…",
+    "collectionEmpty": "No collections"
   },
   "count": "{{count}} titles",
   "searchCount": "{{count}} matches",
@@ -103,7 +106,8 @@
     "clearResume": "Clear resume position",
     "filterYear": "Show titles from {{year}}",
     "filterRating": "Show titles rated {{n}}+",
-    "filterActor": "Show titles with {{name}}"
+    "filterActor": "Show titles with {{name}}",
+    "browseCollection": "Browse collection →"
   },
   "playable": {
     "direct": "Plays directly in the browser",