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

ROADMAP.md

@@ -34,15 +34,15 @@ Goal: improve resilience, code quality, and development experience.
 
 ---
 
-## Phase 3: Scale
+## Phase 3: Scale ✅
 Goal: handle larger data volumes and support real-time ingestion.
 
-- [ ] Replace skip-based pagination with cursor-based (search-after) pagination
-- [ ] Add Prometheus `/metrics` endpoint and a Grafana dashboard
-- [ ] Implement incremental fetch watermarking per source (store last fetch timestamp)
-- [ ] Add webhook endpoints to receive Microsoft Graph change notifications
-- [ ] Evaluate Elasticsearch or Azure Cognitive Search for advanced full-text search
-- [ ] Add request ID / correlation ID middleware for distributed tracing
+- [x] Replace skip-based pagination with cursor-based (search-after) pagination
+- [x] Add Prometheus `/metrics` endpoint and a Grafana dashboard
+- [x] Implement incremental fetch watermarking per source (store last fetch timestamp)
+- [x] Add webhook endpoints to receive Microsoft Graph change notifications
+- [x] Evaluate Elasticsearch or Azure Cognitive Search for advanced full-text search (MongoDB text index sufficient for current scale)
+- [x] Add request ID / correlation ID middleware for distributed tracing
 
 ---