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/PlexInfo.tsx

@@ -693,7 +693,12 @@ export default function PlexInfo({
                 </h2>
                 {onFilter && (
                   <button
-                    onClick={() => onFilter({ collection: col.id, collectionTitle: col.title })}
+                    onClick={() =>
+                      onFilter({
+                        collections: [col.id],
+                        collectionTitles: { [col.id]: col.title },
+                      })
+                    }
                     className="shrink-0 text-xs text-muted hover:text-accent"
                   >
                     {t("plex.info.browseCollection")}