feat(plex): add a movie or show to a collection from its card menu

"Add to collection…" opens a panel beside the card: the title's library's
manual collections tick it in or out, and a name + Enter creates a new one with
the title in it. It used to take the Collections page, a new collection, its
name and a title search, then another search per title. A lean endpoint lists
the collections with whether each holds the title. Admin only, as on the
title's page.

Also: a collection seeded with a show is made as a show collection (Plex type
2). Measured: as a movie one it came out empty and refused the show after.

backend/tests/test_plex_collections_filter.py

@@ -5,6 +5,7 @@ DB lane (`db` fixture) — skipped without Postgres.
 
 from datetime import datetime, timezone
 
+import pytest
 from fastapi.testclient import TestClient
 from sqlalchemy import select
 
@@ -224,8 +225,9 @@ class _CollStub:
         if self.fail_on == name:
             raise PlexError("boom")
 
-    def create_collection(self, section_key, title, item_rating_key):
+    def create_collection(self, section_key, title, item_rating_key, item_type=1):
         self._maybe("create_collection")
+        self.created_type = item_type
         return {"ratingKey": "NEW"}
 
     def add_collection_item(self, rk, item):
@@ -320,6 +322,22 @@ def test_create_writes_through_to_plex(db, monkeypatch, override_dep):
     assert body["editable"] is True  # create marks the new collection editable
 
 
+@pytest.mark.parametrize(("seed", "plex_type"), [("s1", 2), ("m1", 1)])
+def test_create_makes_a_show_collection_as_one(db, monkeypatch, override_dep, seed, plex_type):
+    # Measured on a real Plex: a collection made as a movie one (type 1) in a TV section came out
+    # empty with a show seed, and refused the show when it was added after.
+    lib, user = _seed_editable(db)
+    db.add(PlexShow(rating_key="s1", library_id=lib.id, title="Series"))
+    db.commit()
+    stub = _CollStub()
+    client = _admin_client(db, user, monkeypatch, override_dep, stub, resync=_resync_create)
+    r = client.post(
+        "/api/plex/collections", json={"library": "", "title": "New", "item_rating_key": seed}
+    )
+    assert r.status_code == 200, r.text
+    assert stub.created_type == plex_type
+
+
 def test_create_resync_failure_is_422(db, monkeypatch, override_dep):
     # The collection was created in Plex (wrote=True) but the follow-up resync read failed → 422 with
     # the honest "reconcile on the next sync" detail, not a false "write failed".

backend/tests/test_plex_collections_for_item.py

@@ -0,0 +1,82 @@
+"""A movie or show card's "Add to collection": the collections it can go into, each saying whether the
+title is in it already. DB lane (`db` fixture)."""
+
+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 PlexCollection, PlexItem, PlexLibrary, PlexShow, User
+
+
+def _seed(db):
+    movies = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
+    shows = PlexLibrary(plex_key="2", title="Shows", kind="show", enabled=True)
+    db.add_all([movies, shows])
+    db.flush()
+    enriched = datetime(2024, 1, 1, tzinfo=timezone.utc)
+    db.add_all(
+        [
+            PlexItem(
+                rating_key="m1",
+                title="Film",
+                kind="movie",
+                library_id=movies.id,
+                collection_keys=["cA"],
+                people_enriched_at=enriched,
+            ),
+            PlexItem(
+                rating_key="e1",
+                title="An episode",
+                kind="episode",
+                library_id=shows.id,
+                people_enriched_at=enriched,
+            ),
+            PlexShow(rating_key="s1", title="Series", library_id=shows.id, collection_keys=[]),
+            # The movie library's: two manual ones it can go into, and three it cannot.
+            PlexCollection(rating_key="cA", library_id=movies.id, title="Alpha", editable=True),
+            PlexCollection(rating_key="cB", library_id=movies.id, title="Beta", editable=True),
+            PlexCollection(rating_key="cP", library_id=movies.id, title="Plain", editable=False),
+            PlexCollection(
+                rating_key="cS", library_id=movies.id, title="Smart", editable=True, smart=True
+            ),
+            PlexCollection(
+                rating_key="cI", library_id=movies.id, title="IMDb Top 250", editable=True
+            ),
+            # The show library's own.
+            PlexCollection(rating_key="cT", library_id=shows.id, title="Sci-fi", editable=True),
+        ]
+    )
+    user = User(email="collfor@example.com")
+    db.add(user)
+    db.commit()
+    return user
+
+
+def _get(db, monkeypatch, override_dep, rk):
+    user = _seed(db)
+    monkeypatch.setattr(state, "_configured_cache", True)
+    override_dep(get_db, db.info["request"])
+    override_dep(current_user, lambda: user)
+    return TestClient(app).get(f"/api/plex/collections/for/{rk}")
+
+
+def test_a_movie_lists_its_librarys_manual_collections_and_which_hold_it(
+    db, monkeypatch, override_dep
+):
+    resp = _get(db, monkeypatch, override_dep, "m1")
+    assert resp.status_code == 200
+    got = [(c["id"], c["contains"]) for c in resp.json()["collections"]]
+    assert got == [("cA", True), ("cB", False)]
+
+
+def test_a_show_lists_its_own_librarys(db, monkeypatch, override_dep):
+    resp = _get(db, monkeypatch, override_dep, "s1")
+    assert [(c["id"], c["contains"]) for c in resp.json()["collections"]] == [("cT", False)]
+
+
+def test_an_episode_or_an_unknown_key_is_not_a_collection_member(db, monkeypatch, override_dep):
+    assert _get(db, monkeypatch, override_dep, "e1").status_code == 404