fix: advertise retry delay on context timeout (#40)

tests/test_context_timeout.py

@@ -27,6 +27,7 @@ async def test_context_returns_fallback_when_gitea_exceeds_deadline(monkeypatch)
     response = await main.context()
 
     assert response.status_code == 200
+    assert response.headers["retry-after"] == "1"
     assert b'"repos":[]' in response.body
     assert b'"error":"Gitea context request timed out after 0.01s"' in response.body
     assert cancelled.is_set()