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/routes/webhooks.py

@@ -0,0 +1,32 @@
+import structlog
+from fastapi import APIRouter, Request, Response
+
+router = APIRouter()
+logger = structlog.get_logger("aoc.webhooks")
+
+
+@router.post("/webhooks/graph")
+async def graph_webhook(request: Request):
+    """
+    Receive Microsoft Graph change notifications.
+    Handles the validation handshake by echoing validationToken.
+    """
+    validation_token = request.query_params.get("validationToken")
+    if validation_token:
+        return Response(content=validation_token, media_type="text/plain")
+
+    try:
+        body = await request.json()
+    except Exception as exc:
+        logger.warning("Invalid webhook payload", error=str(exc))
+        return Response(status_code=400)
+
+    for notification in body.get("value", []):
+        logger.info(
+            "Received Graph notification",
+            change_type=notification.get("changeType"),
+            resource=notification.get("resource"),
+            client_state=notification.get("clientState"),
+        )
+
+    return {"status": "accepted"}