refactor: 移除 Ops 监控模块

移除未完成的运维监控功能，简化系统架构：
- 删除 ops_handler, ops_service, ops_repo 等后端代码
- 删除 ops 相关数据库迁移文件
- 删除前端 OpsDashboard 页面和 API

backend/migrations/022_enable_ops_alert_webhook.sql

--- Enable webhook notifications for rules with webhook_url configured
-
-UPDATE ops_alert_rules
-SET notify_webhook = TRUE
-WHERE webhook_url IS NOT NULL
-  AND webhook_url <> ''
-  AND notify_webhook IS DISTINCT FROM TRUE;

backend/migrations/023_ops_metrics_request_counts.sql

--- Add request counts to ops_system_metrics so the UI/alerts can distinguish "no traffic" from "healthy".
-
-ALTER TABLE ops_system_metrics
-    ADD COLUMN IF NOT EXISTS request_count BIGINT NOT NULL DEFAULT 0,
-    ADD COLUMN IF NOT EXISTS success_count BIGINT NOT NULL DEFAULT 0,
-    ADD COLUMN IF NOT EXISTS error_count BIGINT NOT NULL DEFAULT 0;