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/api/channelMonitor.ts

@@ -0,0 +1,74 @@
+/**
+ * User-facing Channel Monitor API endpoints
+ * Read-only views for end users to inspect channel availability/status.
+ */
+
+import { apiClient } from './client'
+import type { Provider, MonitorStatus } from './admin/channelMonitor'
+
+export type { Provider, MonitorStatus } from './admin/channelMonitor'
+
+export interface UserMonitorExtraModel {
+  model: string
+  status: MonitorStatus
+  latency_ms: number | null
+}
+
+export interface UserMonitorView {
+  id: number
+  name: string
+  provider: Provider
+  group_name: string
+  primary_model: string
+  primary_status: MonitorStatus
+  primary_latency_ms: number | null
+  availability_7d: number
+  extra_models: UserMonitorExtraModel[]
+}
+
+export interface UserMonitorListResponse {
+  items: UserMonitorView[]
+}
+
+export interface UserMonitorModelDetail {
+  model: string
+  latest_status: MonitorStatus
+  latest_latency_ms: number | null
+  availability_7d: number
+  availability_15d: number
+  availability_30d: number
+  avg_latency_7d_ms: number | null
+}
+
+export interface UserMonitorDetail {
+  id: number
+  name: string
+  provider: Provider
+  group_name: string
+  models: UserMonitorModelDetail[]
+}
+
+/**
+ * List all monitor views available to the current user.
+ */
+export async function list(options?: { signal?: AbortSignal }): Promise<UserMonitorListResponse> {
+  const { data } = await apiClient.get<UserMonitorListResponse>('/channel-monitors', {
+    signal: options?.signal,
+  })
+  return data
+}
+
+/**
+ * Get detailed status (multi-window availability + latency) for a single monitor.
+ */
+export async function status(id: number): Promise<UserMonitorDetail> {
+  const { data } = await apiClient.get<UserMonitorDetail>(`/channel-monitors/${id}/status`)
+  return data
+}
+
+export const channelMonitorUserAPI = {
+  list,
+  status,
+}
+
+export default channelMonitorUserAPI