feat: implement Phase 3 scaling

- Replace skip-based pagination with cursor-based pagination (timestamp|_id cursors)
- Add Prometheus /metrics endpoint with request latency, fetch volume, and error counters
- Implement incremental fetch watermarking per source (watermarks collection in MongoDB)
- Add Graph change notification webhook endpoint (/api/webhooks/graph)
- Add correlation ID middleware for distributed tracing (x-request-id header)
- Update frontend to use cursor-based pagination with Prev/Next navigation
- Update tests for cursor pagination, metrics, webhooks, and watermark mocking

backend/tests/conftest.py

@@ -12,13 +12,22 @@ def mock_events_collection():
 
 
 @pytest.fixture(scope="function")
-def client(mock_events_collection, monkeypatch):
-    # Patch the collection in all modules that import it before the app is imported
+def mock_watermarks_collection():
+    client = mongomock.MongoClient()
+    db = client["micro_soc"]
+    coll = db["watermarks"]
+    return coll
+
+
+@pytest.fixture(scope="function")
+def client(mock_events_collection, mock_watermarks_collection, monkeypatch):
     monkeypatch.setattr("database.events_collection", mock_events_collection)
     monkeypatch.setattr("routes.fetch.events_collection", mock_events_collection)
     monkeypatch.setattr("routes.events.events_collection", mock_events_collection)
+    monkeypatch.setattr("watermark.watermarks_collection", mock_watermarks_collection)
+    monkeypatch.setattr("routes.fetch.get_watermark", lambda source: None)
+    monkeypatch.setattr("routes.fetch.set_watermark", lambda source, ts: None)
     monkeypatch.setattr("auth.AUTH_ENABLED", False)
-    # Patch health check db.command so it doesn't need a real MongoDB server
     monkeypatch.setattr("database.db.command", lambda cmd: {"ok": 1} if cmd == "ping" else {})
 
     from main import app