fix(tests): use dependency_overrides for get_task_executor, not patch()

FastAPI resolves Depends(get_task_executor) against the function object it
captured when each route was decorated, at import time. patch('src.main.
get_task_executor') therefore never reaches an already-registered route —
confirmed empirically with a minimal FastAPI app before touching this repo's
tests. Every one of these tests fell through to the real dependency, which
called the real psycopg2.connect() and failed on postgres-shared's DNS (a
Docker-network name unreachable from a host process either way, so this
reproduced identically with and without host networking).

app.dependency_overrides is FastAPI's own supported mechanism for this, and
was already used correctly elsewhere in this suite (test_task_delete.py).
Added a shared override_task_executor fixture in conftest.py and switched
every get_task_executor patch() call site in test_api_comprehensive.py to it.

Source (src/main.py) is unchanged — Depends() is the idiomatic, correct
pattern; the tests were using an ineffective substitute for it.

tests/conftest.py

@@ -20,7 +20,7 @@ os.environ["GITEA_USER"] = "test-librarian"
 os.environ["GITEA_PASSWORD"] = "test-gitea-token"
 os.environ["REDIS_HOST"] = "redis-shared"
 
-from src.main import app
+from src.main import app, get_task_executor
 from src.config import Settings, get_settings
 
 
@@ -44,6 +44,22 @@ def auth_headers(api_key: str) -> dict:
     return {"Authorization": f"Bearer {api_key}"}
 
 
+# FastAPI resolves `Depends(get_task_executor)` against the function object it
+# captured when each route was decorated, at import time. `unittest.mock.patch`
+# on the module attribute `src.main.get_task_executor` therefore never reaches
+# an already-registered route — the route keeps calling the original function.
+# `app.dependency_overrides` is FastAPI's own supported mechanism for this
+# (already used correctly in test_task_delete.py); this fixture centralizes it
+# so call sites just need the executor mock they want installed.
+@pytest.fixture
+def override_task_executor() -> Generator[MagicMock, None, None]:
+    """Install a mock TaskExecutor as the live dependency for this test only."""
+    mock_executor = MagicMock()
+    app.dependency_overrides[get_task_executor] = lambda: mock_executor
+    yield mock_executor
+    app.dependency_overrides.pop(get_task_executor, None)
+
+
 # Synchronous test client
 @pytest.fixture
 def client() -> Generator[TestClient, None, None]: