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_person.py

@@ -25,17 +25,14 @@ from app.plex.query import content_libs
 NAME = "Robert De Niro"
 
 
-def _get(db, user, monkeypatch, path):
+def _get(db, user, monkeypatch, override_dep, path):
     monkeypatch.setattr(state, "_configured_cache", True)  # pass the request gate
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: user
-    try:
-        return TestClient(app).get(path)  # no lifespan (skip scheduler/HLS sweep)
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(current_user, lambda: user)
+    return TestClient(app).get(path)  # no lifespan (skip scheduler/HLS sweep)
 
 
-def test_person_detail_returns_cast_and_director_credits(db, monkeypatch):
+def test_person_detail_returns_cast_and_director_credits(db, monkeypatch, override_dep):
     movies_lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
     shows_lib = PlexLibrary(plex_key="2", title="Shows", kind="show", enabled=True)
     off_lib = PlexLibrary(plex_key="3", title="Off", kind="movie", enabled=False)
@@ -89,7 +86,7 @@ def test_person_detail_returns_cast_and_director_credits(db, monkeypatch):
     db.add(PlexState(user_id=user.id, item_id=heat.id, status="watched"))
     db.commit()
 
-    resp = _get(db, user, monkeypatch, f"/api/plex/person/{NAME.replace(' ', '%20')}")
+    resp = _get(db, user, monkeypatch, override_dep, f"/api/plex/person/{NAME.replace(' ', '%20')}")
     assert resp.status_code == 200, resp.text
     body = resp.json()
 
@@ -106,7 +103,7 @@ def test_person_detail_returns_cast_and_director_credits(db, monkeypatch):
     assert body["shows"][0]["type"] == "show"
 
 
-def test_person_detail_unknown_name_is_empty(db, monkeypatch):
+def test_person_detail_unknown_name_is_empty(db, monkeypatch, override_dep):
     lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
     db.add(lib)
     db.flush()
@@ -123,7 +120,7 @@ def test_person_detail_unknown_name_is_empty(db, monkeypatch):
     db.add(user)
     db.commit()
 
-    resp = _get(db, user, monkeypatch, "/api/plex/person/Ghost%20Actor")
+    resp = _get(db, user, monkeypatch, override_dep, "/api/plex/person/Ghost%20Actor")
     assert resp.status_code == 200, resp.text
     body = resp.json()
     # An unknown name has no plex_people row → no bio, no titles (E5 S6 added the bio fields, all null).