refactor(observability): restructure into service-owned packages

tests/integration/test_ingestion_api.py

@@ -5,6 +5,7 @@ import pytest
 from fastapi.testclient import TestClient
 
 from core.api.main import create_app
+from core.api.observability.stats import WorkerStatsUnavailable
 from core.api.publisher import (
     IngestionQueueFull,
     KafkaPublishError,
@@ -327,3 +328,31 @@ def test_successful_request_increments_api_metric(
     )
 
 
+def test_stats_endpoint_when_worker_unavailable(
+    publisher: FakePublisher,
+) -> None:
+    class FailingWorkerStatsClient:
+        async def get_stats(self) -> dict[str, Any]:
+            raise WorkerStatsUnavailable(
+                "Worker stats endpoint unreachable"
+            )
+
+        async def close(self) -> None:
+            pass
+
+    app = create_app(
+        publisher=publisher,
+        worker_stats_client=FailingWorkerStatsClient(),
+    )
+
+    with TestClient(app) as test_client:
+        response = test_client.get("/stats")
+
+        assert response.status_code == 503
+
+        body = response.json()
+        assert body["error"] == "worker_stats_unavailable"
+        assert "Worker stats endpoint unreachable" in body["detail"]
+
+
+