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

internal/db/migrator/goose/clickhouse/202607020002_create_node_traffic_hourly_mv.sql

@@ -0,0 +1,28 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS of_node_traffic_hourly
+(
+    node_id               String,
+    hour                  DateTime,
+    request_count         UInt64,
+    error_count           UInt64,
+    unique_visitor_count  UInt64
+)
+ENGINE = SummingMergeTree()
+PARTITION BY toYYYYMM(hour)
+ORDER BY (node_id, hour);
+
+CREATE MATERIALIZED VIEW IF NOT EXISTS of_node_traffic_hourly_mv
+TO of_node_traffic_hourly
+AS
+SELECT
+    node_id,
+    toStartOfHour(window_ended_at) AS hour,
+    sum(request_count) AS request_count,
+    sum(error_count) AS error_count,
+    sum(unique_visitor_count) AS unique_visitor_count
+FROM of_node_request_reports
+GROUP BY node_id, hour;
+
+-- +goose Down
+DROP VIEW IF EXISTS of_node_traffic_hourly_mv;
+DROP TABLE IF EXISTS of_node_traffic_hourly;