feat: auto-cleanup metrics older than 60 days

- MariaDB event scheduler: daily cleanup at 03:00
- Production: event created, event_scheduler=ON, config in /etc/mysql/mariadb.conf.d/
- Docker: --event-scheduler=ON in db service command
- Migration 008: creates event on fresh installs
- 20 servers x 1 month ≈ 1GB (vs unlimited growth)

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

docker/docker-compose.yml

@@ -41,6 +41,7 @@ services:
       --character-set-server=utf8mb4
       --collation-server=utf8mb4_unicode_ci
       --max-connections=100
+      --event-scheduler=ON
     volumes:
       - db_data:/var/lib/mysql
     healthcheck: