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_lang.py

@@ -56,54 +56,55 @@ def _seed(db):
     return user
 
 
-def _get(db, user, monkeypatch, path):
+def _get(db, user, monkeypatch, override_dep, path):
     monkeypatch.setattr(
         state, "_configured_cache", True
     )  # the request gate 503s an unconfigured app
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: user
-    try:
-        client = TestClient(app)  # no context manager: skip lifespan
-        return client.get(path)
-    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
+    return client.get(path)
 
 
 def _titles(resp):
     return {i["title"] for i in resp.json()["items"]}
 
 
-def test_library_serves_active_language_titles(db, monkeypatch):
+def test_library_serves_active_language_titles(db, monkeypatch, override_dep):
     user = _seed(db)
-    hu = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&lang=hu")
-    en = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&lang=en")
+    hu = _get(db, user, monkeypatch, override_dep, "/api/plex/library?scope=movie&lang=hu")
+    en = _get(db, user, monkeypatch, override_dep, "/api/plex/library?scope=movie&lang=en")
     # HU serves the twin's Hungarian title; the EN-only row falls back to its English title.
     assert _titles(hu) == {"Magyar Cím", "EN Only"}
     assert _titles(en) == {"English Title", "EN Only"}
 
 
-def test_facets_vocabulary_is_language_specific(db, monkeypatch):
+def test_facets_vocabulary_is_language_specific(db, monkeypatch, override_dep):
     user = _seed(db)
-    hu = _get(db, user, monkeypatch, "/api/plex/facets?scope=movie&lang=hu")
-    en = _get(db, user, monkeypatch, "/api/plex/facets?scope=movie&lang=en")
+    hu = _get(db, user, monkeypatch, override_dep, "/api/plex/facets?scope=movie&lang=hu")
+    en = _get(db, user, monkeypatch, override_dep, "/api/plex/facets?scope=movie&lang=en")
     hu_genres = {g["value"] for g in hu.json()["genres"]}
     en_genres = {g["value"] for g in en.json()["genres"]}
     assert "Dráma" in hu_genres and "Drama" not in hu_genres
     assert "Drama" in en_genres and "Dráma" not in en_genres
 
 
-def test_genre_filter_matches_active_language_vocabulary(db, monkeypatch):
+def test_genre_filter_matches_active_language_vocabulary(db, monkeypatch, override_dep):
     user = _seed(db)
     # The HU genre chip filters the HU column; the same string against the EN column matches nothing.
-    hit = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&lang=hu&genres=Dráma")
-    miss = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&lang=en&genres=Dráma")
+    hit = _get(
+        db, user, monkeypatch, override_dep, "/api/plex/library?scope=movie&lang=hu&genres=Dráma"
+    )
+    miss = _get(
+        db, user, monkeypatch, override_dep, "/api/plex/library?scope=movie&lang=en&genres=Dráma"
+    )
     assert _titles(hit) == {"Magyar Cím"}
     assert miss.json()["items"] == []
 
 
-def test_item_detail_language_and_rating_split(db, monkeypatch):
+def test_item_detail_language_and_rating_split(db, monkeypatch, override_dep):
     user = _seed(db)
-    resp = _get(db, user, monkeypatch, "/api/plex/item/m1?lang=hu")
+    resp = _get(db, user, monkeypatch, override_dep, "/api/plex/item/m1?lang=hu")
     body = resp.json()
     assert body["title"] == "Magyar Cím"
     assert body["summary"] == "Magyar összefoglaló."
@@ -112,15 +113,15 @@ def test_item_detail_language_and_rating_split(db, monkeypatch):
     assert body["rating"] == 6.5
 
 
-def test_hu_search_still_finds_en_only_titles(db, monkeypatch):
+def test_hu_search_still_finds_en_only_titles(db, monkeypatch, override_dep):
     user = _seed(db)
     # m2 ("EN Only") has no HU twin, so search_vector_hu carries no title tokens for it. HU-mode search
     # must still match it via the EN vector (vector_match's union) — findable by its English title.
-    resp = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&lang=hu&q=Only")
+    resp = _get(db, user, monkeypatch, override_dep, "/api/plex/library?scope=movie&lang=hu&q=Only")
     assert "EN Only" in _titles(resp)
 
 
-def test_rating_filter_prefers_imdb_over_audience(db, monkeypatch):
+def test_rating_filter_prefers_imdb_over_audience(db, monkeypatch, override_dep):
     lib = PlexLibrary(plex_key="1", title="Movies", kind="movie", enabled=True)
     db.add(lib)
     db.flush()
@@ -142,7 +143,7 @@ def test_rating_filter_prefers_imdb_over_audience(db, monkeypatch):
     user = User(email="rating@example.com")
     db.add(user)
     db.commit()
-    hit = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&rating_min=7")
+    hit = _get(db, user, monkeypatch, override_dep, "/api/plex/library?scope=movie&rating_min=7")
     assert "High IMDb" in _titles(hit)
-    miss = _get(db, user, monkeypatch, "/api/plex/library?scope=movie&rating_min=9")
+    miss = _get(db, user, monkeypatch, override_dep, "/api/plex/library?scope=movie&rating_min=9")
     assert "High IMDb" not in _titles(miss)