perf(clickhouse): Phase 2 legacy governance — TTL cleanup, unified pool, MV, ops API

- Replace retention ALTER DELETE with MATERIALIZE TTL; use TRUNCATE for delete-all
- Remove GORM ClickHouse pool; migrate user access log reads to ChConn
- Drop query-side trim(remote_addr); enable wait_for_async_insert=1
- Add of_node_traffic_hourly MV and dashboard traffic trend fallback
- Add GET /admin/status/clickhouse operational metrics endpoint

docs/plan/clickhouse-cpu-optimization.md

@@ -1,7 +1,7 @@
 # ClickHouse CPU 性能优化计划
 
 > PLAN_ID: `63ba981b`  
-> 状态: 已完成  
+> 状态: 已完成（含 Phase 2 遗留治理）  
 > 目标: 完成 P0–P2 优化，降低 ClickHouse CPU 占用
 
 ## 背景