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/app/plex/client.py

@@ -167,12 +167,16 @@ class PlexClient:
         keys = ",".join(str(k) for k in rating_keys)
         return f"server://{self.machine_id}/com.plexapp.plugins.library/library/metadata/{keys}"
 
-    def create_collection(self, section_key: str, title: str, item_rating_key: str) -> dict:
-        """Create a (dumb) collection seeded with one item; returns its Metadata (incl. ratingKey)."""
+    def create_collection(
+        self, section_key: str, title: str, item_rating_key: str, item_type: int = 1
+    ) -> dict:
+        """Create a (dumb) collection seeded with one item; returns its Metadata (incl. ratingKey).
+        `item_type` is the Plex metadata type of its members — 1 movie, 2 show. Measured: a collection
+        made as type 1 in a TV section came out EMPTY with a show seed, and refused the show after."""
         uri = quote(self._metadata_uri(item_rating_key), safe="")
         mc = self._write(
             "POST",
-            f"/library/collections?type=1&smart=0&sectionId={section_key}"
+            f"/library/collections?type={item_type}&smart=0&sectionId={section_key}"
             f"&title={quote(title, safe='')}&uri={uri}",
         )
         items = mc.get("Metadata", []) or []

backend/app/routes/plex/collections.py

@@ -176,6 +176,40 @@ def collections(
     }
 
 
+@router.get("/collections/for/{item_rating_key}")
+def collections_for_item(
+    item_rating_key: str,
+    user: User = Depends(current_user),
+    db: Session = Depends(get_db),
+) -> dict:
+    """What a movie or show card's "Add to collection" lists: the editable collections of the title's
+    library (a collection lives in one library, and only a manual one can take a title — the same
+    `can_edit` the editor offers), each saying whether the title is in it already, by name."""
+    movie = by_rating_key(db, PlexItem, item_rating_key)
+    title: PlexItem | PlexShow | None = (
+        movie if movie is not None and movie.kind == "movie" else None
+    ) or by_rating_key(db, PlexShow, item_rating_key)
+    if title is None:
+        raise not_found("item")
+    member_of = set(title.collection_keys or [])
+    rows = (
+        db.execute(
+            select(PlexCollection)
+            .where(PlexCollection.library_id == title.library_id)
+            .order_by(collated(PlexCollection.title, lang_for_user(user)))
+        )
+        .scalars()
+        .all()
+    )
+    return {
+        "collections": [
+            {**_collection_card(c), "contains": c.rating_key in member_of}
+            for c in rows
+            if _collection_editable(c)
+        ]
+    }
+
+
 class Member(NamedTuple):
     """One collection member flattened to everything the Sort control can order by. The page's two
     legs are different tables (movies and shows), so there is no single query to ORDER BY — the
@@ -407,13 +441,14 @@ def create_collection(
     seed = body.item_rating_key
     if not title or not seed:
         raise refused(400, "collection_fields_required")
+    seed_show = by_rating_key(db, PlexShow, seed)
     if library:
         lib = (
             db.execute(select(PlexLibrary).where(PlexLibrary.plex_key == library)).scalars().first()
         )
     else:
         # The "New collection" flow only knows the seed title → derive its library.
-        seed_item = by_rating_key(db, PlexItem, seed) or by_rating_key(db, PlexShow, seed)
+        seed_item = by_rating_key(db, PlexItem, seed) or seed_show
         lib = db.get(PlexLibrary, seed_item.library_id) if seed_item else None
     if lib is None:
         raise not_found("library")
@@ -421,7 +456,8 @@ def create_collection(
     wrote = False
     try:
         with PlexClient(db) as plex:
-            new = plex.create_collection(lib.plex_key, title, seed)
+            # A show collection is made as one (Plex type 2): as a movie one it stayed empty.
+            new = plex.create_collection(lib.plex_key, title, seed, 2 if seed_show else 1)
             wrote = True
             col = plex_sync.resync_collection(db, plex, lib, str(new.get("ratingKey")))
             col.editable = True

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