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).

backend/tests/test_plex_collections_filter.py

@@ -0,0 +1,72 @@
+"""Piece 1a: the Plex collection filter is multi-select with any/all combine (a title can belong to
+several collections). `collections=A,B` = ANY (OR, default); `&collection_mode=all` = ALL (AND).
+DB lane (`db` fixture) — skipped without Postgres.
+"""
+from datetime import datetime, timezone
+
+from fastapi.testclient import TestClient
+
+import app.state as state
+from app.auth import current_user
+from app.db import get_db
+from app.main import app
+from app.models import PlexItem, PlexLibrary, User
+
+
+def _seed(db):
+    """Three movies: one in collection A, one in B, one in both — so any/all are distinguishable."""
+    lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
+    db.add(lib)
+    db.flush()
+    common = dict(
+        library_id=lib.id,
+        kind="movie",
+        people_enriched_at=datetime(2024, 1, 1, tzinfo=timezone.utc),
+    )
+    db.add_all([
+        PlexItem(rating_key="mA", title="Only A", collection_keys=["A"],
+                 added_at=datetime(2024, 1, 1, tzinfo=timezone.utc), **common),
+        PlexItem(rating_key="mB", title="Only B", collection_keys=["B"],
+                 added_at=datetime(2024, 1, 2, tzinfo=timezone.utc), **common),
+        PlexItem(rating_key="mAB", title="A and B", collection_keys=["A", "B"],
+                 added_at=datetime(2024, 1, 3, tzinfo=timezone.utc), **common),
+    ])
+    user = User(email="plexcoll@example.com")
+    db.add(user)
+    db.commit()
+    return user
+
+
+def _get(db, user, monkeypatch, path):
+    monkeypatch.setattr(state, "_configured_cache", True)
+    app.dependency_overrides[get_db] = lambda: db
+    app.dependency_overrides[current_user] = lambda: user
+    try:
+        return TestClient(app).get(path)
+    finally:
+        app.dependency_overrides.clear()
+
+
+def _titles(resp):
+    return {i["title"] for i in resp.json()["items"]}
+
+
+def test_single_collection_filters_to_its_members(db, monkeypatch):
+    user = _seed(db)
+    resp = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&collections=A")
+    assert _titles(resp) == {"Only A", "A and B"}
+
+
+def test_multiple_collections_any_is_the_union(db, monkeypatch):
+    user = _seed(db)
+    resp = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&collections=A,B")
+    assert _titles(resp) == {"Only A", "Only B", "A and B"}
+
+
+def test_multiple_collections_all_is_the_intersection(db, monkeypatch):
+    user = _seed(db)
+    resp = _get(
+        db, user, monkeypatch,
+        "/api/plex/library?scope=movie&collections=A,B&collection_mode=all",
+    )
+    assert _titles(resp) == {"A and B"}