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/graph/audit_logs.py

@@ -5,10 +5,10 @@ from graph.resolve import resolve_directory_object, resolve_service_principal_ow
 from utils.http import get_with_retry
 
 
-def fetch_audit_logs(hours=24, max_pages=50):
+def fetch_audit_logs(hours: int = 24, since: str | None = None, max_pages: int = 50):
     """Fetch paginated directory audit logs from Microsoft Graph and enrich with resolved names."""
     token = get_access_token()
-    start_time = (datetime.utcnow() - timedelta(hours=hours)).isoformat() + "Z"
+    start_time = since or (datetime.utcnow() - timedelta(hours=hours)).isoformat() + "Z"
     next_url = (
         "https://graph.microsoft.com/v1.0/"
         f"auditLogs/directoryAudits?$filter=activityDateTime ge {start_time}"