Add interactive ClickHouse cluster topology visualization

Features:
- Backend: New /api/clusters/topology endpoint with comprehensive topology data
  - Queries system.clusters, system.replicas, system.tables, and system.parts
  - Returns cluster structure, replication status, and table metadata

- Frontend: Interactive graph visualization using @antv/graphin
  - Three view modes: Cluster Topology, Tables & Databases, Combined View
  - Hierarchical graph showing clusters → shards → replicas
  - Database and table visualization with replication indicators
  - Color-coded health status (green: healthy, yellow: slowdowns, red: errors)
  - Click-to-expand node details in sidebar drawer
  - Supports multiple clusters with dropdown selector

- Dependencies: Added @antv/g6 (5.0.50) and @antv/graphin (3.0.5) for graph rendering

This view enables ops teams to quickly understand their ClickHouse cluster
architecture, identify unhealthy nodes, and visualize data distribution.

Author: claude

frontend/package.json

@@ -5,6 +5,8 @@
         "@ant-design/charts": "^1.4.2",
         "@ant-design/icons": "^5.2.0",
         "@ant-design/plots": "^2.1.12",
+        "@antv/g6": "^5.0.50",
+        "@antv/graphin": "^3.0.5",
         "antd": "^5.13.2",
         "prismjs": "^1.29.0",
         "react": "^18.2.0",