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/i18n/locales/en.ts

@@ -245,6 +245,7 @@ export default {
   // Common
   common: {
     loading: 'Loading...',
+    submitting: 'Submitting...',
     justNow: 'just now',
     save: 'Save',
     saved: 'Saved successfully',
@@ -363,7 +364,11 @@ export default {
     orderManagement: 'Orders',
     paymentDashboard: 'Payment Dashboard',
     paymentConfig: 'Payment Config',
-    paymentPlans: 'Plans'
+    paymentPlans: 'Plans',
+    channelManagement: 'Channels',
+    channelPricing: 'Channel Pricing',
+    channelMonitor: 'Channel Monitor',
+    channelStatus: 'Channel Status',
   },
 
   // Auth
@@ -846,6 +851,58 @@ export default {
     userAgent: 'User-Agent'
   },
 
+  // Shared keys for channel monitor (admin + user views)
+  monitorCommon: {
+    status: {
+      operational: 'Operational',
+      degraded: 'Degraded',
+      failed: 'Failed',
+      error: 'Error',
+      unknown: '-'
+    },
+    providers: {
+      openai: 'OpenAI',
+      anthropic: 'Anthropic',
+      gemini: 'Gemini'
+    },
+    extraModelsHeader: 'Extra Models',
+    extraModelsEmpty: 'No extra models',
+    latencyEmpty: '-'
+  },
+
+  // Channel Status (user-facing read-only view)
+  channelStatus: {
+    title: 'Channel Status',
+    description: 'Inspect channel availability, latency and recent status',
+    searchPlaceholder: 'Search channels...',
+    allProviders: 'All Providers',
+    loadError: 'Failed to load channel status',
+    detailLoadError: 'Failed to load channel detail',
+    detailTitle: 'Channel Detail',
+    closeDetail: 'Close',
+    columns: {
+      name: 'Name',
+      provider: 'Provider',
+      groupName: 'Group',
+      primaryModel: 'Primary Model',
+      availability7d: '7d Availability',
+      latency: 'Latency (ms)'
+    },
+    detailColumns: {
+      model: 'Model',
+      latestStatus: 'Latest Status',
+      latestLatency: 'Latest Latency (ms)',
+      availability7d: '7d Availability',
+      availability15d: '15d Availability',
+      availability30d: '30d Availability',
+      avgLatency7d: '7d Avg Latency (ms)'
+    },
+    empty: {
+      title: 'No channels available',
+      description: 'No monitored channels have been configured yet.'
+    }
+  },
+
   // Redeem
   redeem: {
     title: 'Redeem Code',
@@ -2014,6 +2071,69 @@ export default {
       }
     },
 
+    // Channel Monitor
+    channelMonitor: {
+      title: 'Channel Monitor',
+      description: 'Monitor channel availability, latency and status',
+      searchPlaceholder: 'Search monitor name...',
+      allProviders: 'All Providers',
+      allStatus: 'All Status',
+      enabledFilter: 'Enabled',
+      onlyEnabled: 'Enabled only',
+      onlyDisabled: 'Disabled only',
+      createButton: 'Create Monitor',
+      createTitle: 'Create Channel Monitor',
+      editTitle: 'Edit Channel Monitor',
+      runNow: 'Run Now',
+      runSuccess: 'Check completed',
+      runFailed: 'Check failed',
+      apiKeyDecryptFailed: 'API Key decryption failed. Please re-edit this monitor with a fresh key.',
+      createSuccess: 'Monitor created',
+      updateSuccess: 'Monitor updated',
+      deleteSuccess: 'Monitor deleted',
+      loadError: 'Failed to load monitors',
+      deleteConfirm: 'Are you sure you want to delete monitor "{name}"? This action cannot be undone.',
+      nameRequired: 'Please enter a monitor name',
+      primaryModelRequired: 'Please enter a primary model',
+      columns: {
+        name: 'Name',
+        provider: 'Provider',
+        primaryModel: 'Primary Model',
+        availability7d: '7d Availability',
+        latency: 'Latency (ms)',
+        enabled: 'Enabled',
+        actions: 'Actions'
+      },
+      form: {
+        name: 'Name',
+        namePlaceholder: 'Enter monitor name',
+        provider: 'Provider',
+        endpoint: 'Endpoint',
+        endpointPlaceholder: 'https://api.example.com',
+        useCurrentDomain: 'Use current service',
+        apiKey: 'API Key',
+        apiKeyPlaceholder: 'Enter API Key',
+        apiKeyEditPlaceholder: 'Leave blank to keep current key',
+        useMyKey: 'Use my key',
+        selectKeyTitle: 'Select my API Key',
+        selectKeyHint: 'Only your active, non-expired keys are listed.',
+        noActiveKey: 'No active API keys available',
+        primaryModel: 'Primary Model',
+        primaryModelPlaceholder: 'gpt-4o-mini',
+        extraModels: 'Extra Models',
+        extraModelsPlaceholder: 'Press Enter to add extra model',
+        groupName: 'Group Name',
+        groupNamePlaceholder: 'Optional, used to group rows in user view',
+        intervalSeconds: 'Interval (seconds)',
+        intervalSecondsHint: 'Range: 15 - 3600 seconds',
+        enabled: 'Enable monitor',
+        kindRequired: 'Please select a provider'
+      },
+      runResultTitle: 'Check Result',
+      noMonitorsYet: 'No monitors yet',
+      createFirstMonitor: 'Create your first monitor to track channel availability'
+    },
+
     // Subscriptions
     subscriptions: {
       title: 'Subscription Management',