feat(plex): make the Collection filter multi-select with Any/All

A title can belong to several Plex collections, so the collection filter becomes
multi-select combining with Any (OR, default) / All (AND) — the same shape as the
genre/actor filters.

Backend (query.py): `collection` (single) → `collections` CSV + `collection_mode`
on LibraryFilters; meta_filter_conds ORs/ANDs `collection_keys.contains` per id.
Frontend: PlexFilters `collection`/`collectionTitle` → `collections` + optional
`collectionMode` + `collectionTitles` map; CollectionPicker is now a multi-select
Combobox with the any/all match footer; one chip per collection (usePlexActive-
Filters); the info-page "Browse collection" strip unions into the list; applyFilter
now shallow-merges label maps so adding one keeps the others' titles. PlexProvider
migrates a legacy persisted single `collection` into the new shape on read.

Test: test_plex_collections_filter (single / any-union / all-intersection).

frontend/src/components/PlexSidebar.tsx

@@ -69,7 +69,9 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
   // This rail's media source (the Plex library) — names the abstraction the feed and Plex rails
   // share; drives the panel layout key. `FilterRail` owns the customize `editing` state now.
   const source: MediaSourceKind = "plex";
-  const { collectionTitle: _ct, ...facetKey } = filters;
+  // Titles are display-only (chip labels) — keep them OUT of the facet query key so renaming/relabelling
+  // never refetches. collectionMode stays IN the key: any/all changes the result set.
+  const { collectionTitles: _cts, ...facetKey } = filters;
   const lang = plexLang(); // HU/EN genre vocabulary — refetch facets on a language switch
   const facetsQ = useQuery({
     queryKey: qk.plexFacets(scope, show, facetKey, lang),
@@ -141,9 +143,11 @@ export default function PlexSidebar({ layout, setLayout, collapsed, onToggleColl
       title: t("plex.filter.collection"),
       body: (
         <CollectionPicker
-          value={filters.collection}
-          title={filters.collectionTitle}
-          onPick={(id, title) => patch({ collection: id, collectionTitle: title })}
+          values={filters.collections}
+          titles={filters.collectionTitles}
+          mode={filters.collectionMode ?? "any"}
+          onChange={(collections, collectionTitles) => patch({ collections, collectionTitles })}
+          onModeChange={(m) => patch({ collectionMode: m })}
         />
       ),
     },
@@ -373,19 +377,23 @@ function PlaylistPicker() {
   );
 }
 
-// The collection filter (E5 S7 → combobox rework): a single-select combi-input matching the
-// genre/studio pickers, its vocabulary searched SERVER-side (the union can run to hundreds). The
-// generic <Combobox> owns the field/keyboard/ARIA; this wrapper only owns the query→fetch (gated on
-// `open`) and maps collections to options. Picking sets the filter (id + title for the chip); clearing
-// the chip drops it.
+// The collection filter (E5 S7 → combobox; now multi-select): a combi-input matching the genre/studio
+// pickers, its vocabulary searched SERVER-side (the union can run to hundreds). The generic <Combobox>
+// owns the field/keyboard/ARIA + the any/all match footer; this wrapper owns the query→fetch (gated on
+// `open`) and maps collections to options. A title can belong to several collections, so multiple can
+// be picked and combined with Any/All. Titles are carried alongside the ids for the active-filter chips.
 function CollectionPicker({
-  value,
-  title,
-  onPick,
+  values,
+  titles,
+  mode,
+  onChange,
+  onModeChange,
 }: {
-  value: string | null | undefined;
-  title: string | null | undefined;
-  onPick: (id: string | null, title: string | null) => void;
+  values: string[];
+  titles: Record<string, string> | undefined;
+  mode: "any" | "all";
+  onChange: (values: string[], titles: Record<string, string>) => void;
+  onModeChange: (mode: "any" | "all") => void;
 }) {
   const { t } = useTranslation();
   const [open, setOpen] = useState(false);
@@ -401,25 +409,31 @@ function CollectionPicker({
     label: c.title,
     count: c.child_count ?? undefined,
   }));
+  // Combine the fetched labels with the ones already stored so the trip through onChange keeps every
+  // selected collection's title (the fetched list is narrowed by the current search).
+  const labelMap: Record<string, string> = { ...titles };
+  for (const o of options) if (o.label) labelMap[o.value] = o.label;
 
   return (
     <Combobox
       label={t("plex.filter.collection")}
-      multiple={false}
       clientFilter={false} // the server already narrows to `search`
       options={options}
-      selected={value ? [value] : []}
-      selectedLabels={value && title ? { [value]: title } : undefined}
+      selected={values}
+      selectedLabels={titles}
       query={search}
       onQueryChange={setSearch}
       onOpenChange={setOpen}
       loading={q.isFetching}
       placeholder={t("plex.filter.collectionSearch")}
       emptyText={t("plex.filter.collectionEmpty")}
+      matchMode={mode === "all" ? "and" : "or"}
+      onMatchModeChange={(m) => onModeChange(m === "and" ? "all" : "any")}
       onChange={(next) => {
-        const id = next[0] ?? null;
-        const picked = options.find((o) => o.value === id);
-        onPick(id, id ? (picked?.label ?? title ?? id) : null);
+        // Carry only the selected ids' titles forward (drop titles for de-selected ones).
+        const nextTitles: Record<string, string> = {};
+        for (const id of next) nextTitles[id] = labelMap[id] ?? titles?.[id] ?? id;
+        onChange(next, nextTitles);
       }}
     />
   );