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/middleware.py

@@ -0,0 +1,16 @@
+import uuid
+
+import structlog
+from fastapi import Request, Response
+from starlette.middleware.base import BaseHTTPMiddleware, RequestResponseEndpoint
+
+
+class CorrelationIdMiddleware(BaseHTTPMiddleware):
+    """Inject or propagate a correlation ID for every request."""
+
+    async def dispatch(self, request: Request, call_next: RequestResponseEndpoint) -> Response:
+        cid = request.headers.get("x-request-id") or uuid.uuid4().hex
+        structlog.contextvars.bind_contextvars(correlation_id=cid)
+        response = await call_next(request)
+        response.headers["x-request-id"] = cid
+        return response