feat: 完善告警规则配置页面 — 增删改查弹窗 + 修复白屏bug + UpdateRule notify_channels序列化修复

- AlertManager: 新建/编辑告警规则弹窗(规则名称/描述/类型/数据源/表达式/级别/持续时间/通知渠道Switch)
- AlertManager: 修复白屏bug — getRules返回{list,total}未解包导致rules.map崩溃(Frontend Pitfall #1)
- AlertManager: 修复规则列表字段显示 — 原引用不存在的rule.metric/operator/threshold字段,改为rule.source/expression
- AlertManager: 新增规则删除按钮(带确认)
- alert.go: 修复UpdateRule notify_channels序列化bug — 原直接赋值[]string到text列导致500,改用toJSON()
- 同步提交前序未提交改动(WebShell/K8s/Server/Login等组件改进)

backend/go.sum

@@ -49,6 +49,8 @@ github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
 github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/gorilla/securecookie v1.1.1/go.mod h1:ra0sb63/xPlUeL+yeDciTfxMRAA+MP+HVt/4epWDjd4=
 github.com/gorilla/sessions v1.2.1/go.mod h1:dk2InVEVJ0sfLlnXv9EAgkf6ecYs/i80K/zI+bUmuGM=
+github.com/gorilla/websocket v1.5.3 h1:saDtZ6Pbx/0u+bgYQ3q96pZgCzfhKXGPqt7kZ72aNNg=
+github.com/gorilla/websocket v1.5.3/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
 github.com/hashicorp/go-uuid v1.0.2/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=
 github.com/hashicorp/go-uuid v1.0.3 h1:2gKiV6YVmrJ1i2CKKa9obLvRieoRGviZFL26PcT/Co8=
 github.com/hashicorp/go-uuid v1.0.3/go.mod h1:6SBZvOh/SIDV7/2o3Jml5SYk/TvGqwFJ/bN7x4byOro=