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/models/api.py

@@ -1,3 +1,4 @@
+
 from pydantic import BaseModel, ConfigDict
 
 
@@ -23,8 +24,8 @@ class EventItem(BaseModel):
 class PaginatedEventResponse(BaseModel):
     items: list[dict]
     total: int
-    page: int
     page_size: int
+    next_cursor: str | None = None
 
 
 class FilterOptionsResponse(BaseModel):