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_channel_backfill.py

@@ -18,7 +18,7 @@ from app.sync import runner, videos
 _PUB = datetime(2020, 1, 1, tzinfo=timezone.utc)
 
 
-def test_channel_detail_returns_normal_shorts_live_counts(db, monkeypatch):
+def test_channel_detail_returns_normal_shorts_live_counts(db, monkeypatch, override_dep):
     # published_at set so channel_detail skips the YouTube-enrich path (no quota / network in tests).
     ch = Channel(id="UC_nsl", title="N/S/L Channel", published_at=_PUB)
     db.add(ch)
@@ -40,12 +40,9 @@ def test_channel_detail_returns_normal_shorts_live_counts(db, monkeypatch):
 
     # The request gate 503s an unconfigured instance; flip the process-level flag (auto-restored).
     monkeypatch.setattr(state, "_configured_cache", True)
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: me
-    try:
-        resp = TestClient(app).get(f"/api/channels/{ch.id}")
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(current_user, lambda: me)
+    resp = TestClient(app).get(f"/api/channels/{ch.id}")
 
     assert resp.status_code == 200, resp.text
     body = resp.json()
@@ -56,7 +53,7 @@ def test_channel_detail_returns_normal_shorts_live_counts(db, monkeypatch):
 
 
 @pytest.mark.parametrize("hidden", [True, False])
-def test_channel_detail_surfaces_subscription_hidden_flag(db, monkeypatch, hidden):
+def test_channel_detail_surfaces_subscription_hidden_flag(db, monkeypatch, hidden, override_dep):
     # S6: the detail GET must expose the per-subscription `hidden` flag so the channel page can offer
     # the same Unhide toggle the manager has (the flag was previously omitted from the detail dict).
     ch = Channel(id="UC_hid", title="Hidden?", published_at=_PUB)
@@ -68,18 +65,15 @@ def test_channel_detail_surfaces_subscription_hidden_flag(db, monkeypatch, hidde
     db.commit()
 
     monkeypatch.setattr(state, "_configured_cache", True)
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: me
-    try:
-        resp = TestClient(app).get(f"/api/channels/{ch.id}")
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(current_user, lambda: me)
+    resp = TestClient(app).get(f"/api/channels/{ch.id}")
 
     assert resp.status_code == 200, resp.text
     assert resp.json()["hidden"] is hidden
 
 
-def test_channel_detail_hidden_defaults_false_when_not_subscribed(db, monkeypatch):
+def test_channel_detail_hidden_defaults_false_when_not_subscribed(db, monkeypatch, override_dep):
     # No subscription row → the flag can't be True; it must serialize as False, not raise.
     ch = Channel(id="UC_hid_ns", title="Unsub", published_at=_PUB)
     me = User(email="hidns@example.com")
@@ -87,12 +81,9 @@ def test_channel_detail_hidden_defaults_false_when_not_subscribed(db, monkeypatc
     db.commit()
 
     monkeypatch.setattr(state, "_configured_cache", True)
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: me
-    try:
-        resp = TestClient(app).get(f"/api/channels/{ch.id}")
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(current_user, lambda: me)
+    resp = TestClient(app).get(f"/api/channels/{ch.id}")
 
     assert resp.status_code == 200, resp.text
     assert resp.json()["hidden"] is False
@@ -115,33 +106,30 @@ def _setup_channel_and_sub(db, *, email, is_demo=False):
     return ch, user
 
 
-def _patch_deep_requested(db, monkeypatch, user):
+def _patch_deep_requested(db, monkeypatch, override_dep, user):
     calls: list[tuple] = []
     # background.add_task runs after the response in TestClient; record what would be scheduled.
     monkeypatch.setattr(channels_route, "backfill_channel_now", lambda *a, **k: calls.append(a))
     monkeypatch.setattr(state, "_configured_cache", True)
-    app.dependency_overrides[get_db] = lambda: db
-    app.dependency_overrides[current_user] = lambda: user
-    try:
-        resp = TestClient(app).patch("/api/channels/UC_wire", json={"deep_requested": True})
-    finally:
-        app.dependency_overrides.clear()
+    override_dep(get_db, lambda: db)
+    override_dep(current_user, lambda: user)
+    resp = TestClient(app).patch("/api/channels/UC_wire", json={"deep_requested": True})
     return resp, calls
 
 
-def test_deep_request_schedules_immediate_backfill_for_non_demo(db, monkeypatch):
+def test_deep_request_schedules_immediate_backfill_for_non_demo(db, monkeypatch, override_dep):
     ch, me = _setup_channel_and_sub(db, email="wire@example.com")
-    resp, calls = _patch_deep_requested(db, monkeypatch, me)
+    resp, calls = _patch_deep_requested(db, monkeypatch, override_dep, me)
     assert resp.status_code == 200, resp.text
     assert resp.json()["deep_requested"] is True
     # scheduled exactly once, with (channel_id, user_id) — the args backfill_channel_now needs.
     assert calls == [(ch.id, me.id)]
 
 
-def test_deep_request_does_not_schedule_immediate_backfill_for_demo(db, monkeypatch):
+def test_deep_request_does_not_schedule_immediate_backfill_for_demo(db, monkeypatch, override_dep):
     # The SHARED demo account must not be able to kick immediate full backfills (budget drain).
     _ch, demo = _setup_channel_and_sub(db, email="demo@example.com", is_demo=True)
-    resp, calls = _patch_deep_requested(db, monkeypatch, demo)
+    resp, calls = _patch_deep_requested(db, monkeypatch, override_dep, demo)
     assert resp.status_code == 200, resp.text
     assert calls == [], "demo must not schedule an immediate backfill"
     # …and deep_requested is not persisted on the shared demo subscription either.