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/components/PlexBrowse.tsx

@@ -142,6 +142,7 @@ export default function PlexBrowse({ q, onClearSearch, library, show, sort, filt
         id={infoId}
         onBack={sub.back}
         onPlay={() => sub.open({ kind: "player", id: infoId })}
+        onPlayItem={(id) => sub.open({ kind: "player", id })}
         onFilter={(patch) => {
           // Clicking a metadata chip sets that filter and shows the filtered grid. Array filters
           // (genres/people/studios) UNION with what's already set (so you can stack people); scalars
@@ -360,11 +361,13 @@ function PlexInfoView({
   id,
   onBack,
   onPlay,
+  onPlayItem,
   onFilter,
 }: {
   id: string;
   onBack: () => void;
   onPlay: () => void;
+  onPlayItem: (id: string) => void;
   onFilter: (patch: Partial<PlexFilters>) => void;
 }) {
   const { t } = useTranslation();
@@ -384,7 +387,13 @@ function PlexInfoView({
         <p className="p-8 text-muted text-sm">{t("plex.loading")}</p>
       ) : (
         <Suspense fallback={<p className="p-8 text-muted text-sm">{t("plex.loading")}</p>}>
-          <PlexInfo detail={q.data} variant="page" onPlay={onPlay} onFilter={onFilter} />
+          <PlexInfo
+            detail={q.data}
+            variant="page"
+            onPlay={onPlay}
+            onPlayItem={onPlayItem}
+            onFilter={onFilter}
+          />
         </Suspense>
       )}
     </div>