test(be): every dependency override goes through override_dep

22 test files set `app.dependency_overrides[...]` by hand and tore down with a blanket `.clear()`
in a `finally`, which wipes every override rather than the test's own and stops fixtures composing.
They now take the `override_dep` fixture (helpers get it passed in), whose teardown restores each
key's prior value; no clear() carried meaning between two phases of a test. A convention test fails
any test file that writes `dependency_overrides` directly.

backend/tests/test_plex_facets.py

@@ -16,7 +16,7 @@ from app.main import app
 from app.models import PlexItem, PlexLibrary, PlexShow, User
 
 
-def test_facets_returns_studio_vocabulary(db, monkeypatch):
+def test_facets_returns_studio_vocabulary(db, monkeypatch, override_dep):
     """E5 S7: /facets returns the studio vocabulary (distinct studio + count) for the sidebar picker."""
     lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
     db.add(lib)
@@ -33,18 +33,15 @@ def test_facets_returns_studio_vocabulary(db, monkeypatch):
     db.add(user)
     db.commit()
     monkeypatch.setattr(state, "_configured_cache", True)
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: user
-    try:
-        resp = TestClient(app).get("/api/plex/facets?scope=movie")
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(current_user, lambda: user)
+    resp = TestClient(app).get("/api/plex/facets?scope=movie")
     assert resp.status_code == 200, resp.text
     studios = {s["value"]: s["count"] for s in resp.json()["studios"]}
     assert studios == {"Pixar": 2, "A24": 1}
 
 
-def test_facets_survives_json_null_genres(db, monkeypatch):
+def test_facets_survives_json_null_genres(db, monkeypatch, override_dep):
     lib = PlexLibrary(plex_key="1", title="Shows", kind="show", enabled=True)
     db.add(lib)
     db.flush()
@@ -61,13 +58,10 @@ def test_facets_survives_json_null_genres(db, monkeypatch):
     db.execute(text("UPDATE plex_shows SET genres = 'null'::jsonb WHERE rating_key = 's1'"))
     db.commit()
 
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: user
-    try:
-        client = TestClient(app)  # no context-manager: skip lifespan (scheduler/HLS sweep)
-        resp = client.get("/api/plex/facets?scope=show")
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(current_user, lambda: user)
+    client = TestClient(app)  # no context-manager: skip lifespan (scheduler/HLS sweep)
+    resp = client.get("/api/plex/facets?scope=show")
 
     assert resp.status_code == 200, resp.text
     # The scalar-null show contributes no genres — and, crucially, doesn't abort the whole endpoint.