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_hls_file_route.py

@@ -62,14 +62,11 @@ def sessions(monkeypatch):
 
 
 @pytest.fixture
-def client(monkeypatch):
+def client(monkeypatch, override_dep):
     monkeypatch.setattr(state, "_configured_cache", True)
     user = User(id=VIEWER, email="hls@example.com")
-    app.dependency_overrides[current_user] = lambda: user
-    try:
-        yield TestClient(app)
-    finally:
-        app.dependency_overrides.pop(current_user, None)
+    override_dep(current_user, lambda: user)
+    return TestClient(app)
 
 
 def test_a_segment_is_served_from_the_session_the_url_names(client, tmp_path, sessions):