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/watermark.py

@@ -0,0 +1,18 @@
+from database import db
+
+watermarks_collection = db["watermarks"]
+
+
+def get_watermark(source: str) -> str | None:
+    """Return the ISO timestamp of the last successful fetch for a source."""
+    doc = watermarks_collection.find_one({"source": source})
+    return doc.get("last_fetch_time") if doc else None
+
+
+def set_watermark(source: str, timestamp: str):
+    """Persist the latest successful fetch timestamp for a source."""
+    watermarks_collection.update_one(
+        {"source": source},
+        {"$set": {"last_fetch_time": timestamp}},
+        upsert=True,
+    )