feat(monitor): admin channel monitor MVP with SSRF protection and batch aggregation

新增 admin「渠道监控」模块（参考 BingZi-233/check-cx），独立于现有 Channel 体系。
admin 配置 + 后台定时调用上游 LLM chat completions 健康检查 + 所有登录用户只读可见。

后端：
- ent: channel_monitor + channel_monitor_history（AES-256-GCM 加密 api_key）
- service 按职责拆分：service/aggregator/validate/checker/runner/ssrf
- provider strategy map 替代 switch（openai/anthropic/gemini）
- repository batch 聚合（ListLatestForMonitorIDs + ComputeAvailabilityForMonitors）消除 N+1
- runner: ticker(5s) + pond worker pool(5) + inFlight 防并发 + TrySubmit 防雪崩
  + 凌晨 3 点 cron 清理 30 天历史
- SSRF 防护：强制 https + 私网/loopback/云元数据 IP 拒绝（127/8、10/8、172.16/12、
  192.168/16、169.254/16、100.64/10、::1、fc00::/7、fe80::/10）+ DialContext
  在 socket 层防 DNS rebinding
- API key sanitize：擦除 url.Error 与上游响应 body 中的 sk-/sk-ant-/AIza/JWT 模式
- APIKeyDecryptFailed 标志位 + 单 monitor 路径检测，避免空 key 调用上游

handler:
- admin: CRUD + 手动触发 + 历史接口（api_key 脱敏）
- user: 只读列表 + 状态详情（去除 api_key/endpoint）
- ParseChannelMonitorID 共用 + dto.ChannelMonitorExtraModelStatus 共用

前端：
- 路由 /admin/channels/{pricing,monitor} + /monitor（用户只读）
- AppSidebar 父项 expandOnly 支持
- ChannelMonitorView 拆为 8 个子组件 + ChannelStatusView 拆出 detail dialog
- composables/useChannelMonitorFormat + constants/channelMonitor 共享
- i18n monitorCommon namespace 消除 admin/user 两 view 重复

合规：所有文件符合 CLAUDE.md（Go ≤ 500 行 / Vue ≤ 300 行 / 函数 ≤ 30 行）
CI: go build / gofmt / golangci-lint(0 issues) / make test-unit / pnpm build 全绿

frontend/src/constants/channelMonitor.ts

@@ -0,0 +1,35 @@
+/**
+ * Channel monitor shared constants.
+ *
+ * Single source of truth for provider/status string values used by both the
+ * admin (`views/admin/ChannelMonitorView.vue`) and user-facing
+ * (`views/user/ChannelStatusView.vue`) screens, plus the shared composable
+ * `useChannelMonitorFormat`.
+ */
+
+import type { Provider, MonitorStatus } from '@/api/admin/channelMonitor'
+
+export const PROVIDER_OPENAI: Provider = 'openai'
+export const PROVIDER_ANTHROPIC: Provider = 'anthropic'
+export const PROVIDER_GEMINI: Provider = 'gemini'
+
+export const PROVIDERS: readonly Provider[] = [
+  PROVIDER_OPENAI,
+  PROVIDER_ANTHROPIC,
+  PROVIDER_GEMINI,
+]
+
+export const STATUS_OPERATIONAL: MonitorStatus = 'operational'
+export const STATUS_DEGRADED: MonitorStatus = 'degraded'
+export const STATUS_FAILED: MonitorStatus = 'failed'
+export const STATUS_ERROR: MonitorStatus = 'error'
+
+export const MONITOR_STATUSES: readonly MonitorStatus[] = [
+  STATUS_OPERATIONAL,
+  STATUS_DEGRADED,
+  STATUS_FAILED,
+  STATUS_ERROR,
+]
+
+/** Default polling interval (seconds) for new monitors. */
+export const DEFAULT_INTERVAL_SECONDS = 60