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_search_scrape_ingest.py

@@ -96,7 +96,7 @@ def test_scrape_ingest_enrich_miss_keeps_nonnull_placeholder(db):
     assert c.enriched_at is None
 
 
-def test_search_youtube_route_returns_422_when_ingest_fails(db, monkeypatch):
+def test_search_youtube_route_returns_422_when_ingest_fails(db, monkeypatch, override_dep):
     # Route-level guard (B-core-1): an ingest failure with nothing yet collected must surface as a 422
     # + message on this JSON endpoint (the FE hides 5xx behind a generic "connection lost" + auto-retry,
     # burning a second quota-costed search), and the session is rolled back. Here the page fetch succeeds
@@ -145,12 +145,9 @@ def test_search_youtube_route_returns_422_when_ingest_fails(db, monkeypatch):
 
     monkeypatch.setattr(search_mod, "_ingest_candidates", _boom)
 
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[require_human] = lambda: user
-    try:
-        resp = TestClient(app).get("/api/search/youtube?q=test+term")
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(require_human, lambda: user)
+    resp = TestClient(app).get("/api/search/youtube?q=test+term")
 
     assert resp.status_code == 422, resp.text
     assert resp.json()["detail"] == {"code": "youtube", "reason": "unavailable"}