feat(notify): add balance low & account quota notification system

- User balance low notification: email alert when balance drops below
  configurable threshold (user email + verified extra emails)
- Account quota notification: broadcast email to admin-configured
  recipients when daily/weekly/total quota usage exceeds alert threshold
- Admin settings: global enable/disable, default threshold, quota
  notification email list (Email Settings tab)
- User profile: enable/disable, custom threshold, add/remove extra
  notification emails with verification code flow
- Account quota: per-dimension alert toggle and threshold in quota
  control card
- Trigger logic: first-crossing only (old >= threshold && new < threshold
  for balance; old < threshold && new >= threshold for quota), naturally
  prevents duplicate notifications without Redis dedup

frontend/src/i18n/locales/en.ts

@@ -902,6 +902,31 @@ export default {
       sendCode: 'Send Code',
       codeSent: 'Verification code sent to your email',
       sendCodeFailed: 'Failed to send verification code'
+    },
+    balanceNotify: {
+      title: 'Balance Low Notification',
+      description: 'Send email alert when account balance falls below threshold',
+      enabled: 'Enable Balance Low Notification',
+      threshold: 'Custom Threshold',
+      thresholdHint: 'Leave empty to use system default',
+      thresholdPlaceholder: 'Enter amount',
+      systemDefault: 'System Default',
+      extraEmails: 'Extra Notification Emails',
+      noExtraEmails: 'No extra notification emails',
+      enterEmail: 'Enter email address',
+      addEmail: 'Add Email',
+      emailPlaceholder: 'Enter email address',
+      sendCode: 'Send Code',
+      codeSent: 'Verification code sent',
+      codeSentTo: 'Code sent to {email}',
+      enterCode: 'Enter verification code',
+      codePlaceholder: '6-digit code',
+      verify: 'Verify & Add',
+      emailAdded: 'Email added',
+      emailRemoved: 'Email removed',
+      verifySuccess: 'Email added successfully',
+      removeEmail: 'Remove',
+      removeSuccess: 'Email removed',
     }
   },
 
@@ -2228,6 +2253,12 @@ export default {
       },
       quotaLimitAmount: 'Total Limit',
       quotaLimitAmountHint: 'Cumulative spending limit. Does not auto-reset.',
+      quotaNotify: {
+        alert: 'Alert Threshold',
+        enabled: 'Enable Alert',
+        threshold: 'Alert Amount',
+        thresholdPlaceholder: 'Enter alert amount',
+      },
       testConnection: 'Test Connection',
       reAuthorize: 'Re-Authorize',
       refreshToken: 'Refresh Token',
@@ -4593,6 +4624,22 @@ export default {
         supportedTypesHint: 'Comma-separated, e.g. alipay,wxpay',
         refundEnabled: 'Allow Refund',
       },
+      balanceNotify: {
+        title: 'Balance Low Notification',
+        description: 'Send email notification when user balance falls below threshold',
+        enabled: 'Enable Balance Low Notification',
+        threshold: 'Default Threshold',
+        thresholdHint: 'Used when user has not set a custom value',
+        thresholdPlaceholder: 'Enter amount',
+      },
+      quotaNotify: {
+        title: 'Account Quota Notification',
+        description: 'Notify admins when account quota usage reaches alert threshold',
+        emails: 'Notification Emails',
+        emailsHint: 'Leave empty to disable notifications',
+        addEmail: 'Add Email',
+        emailPlaceholder: 'Enter email address',
+      },
       smtp: {
         title: 'SMTP Settings',
         description: 'Configure email sending for verification codes',

frontend/src/i18n/locales/zh.ts

@@ -906,6 +906,31 @@ export default {
       sendCode: '发送验证码',
       codeSent: '验证码已发送到您的邮箱',
       sendCodeFailed: '发送验证码失败'
+    },
+    balanceNotify: {
+      title: '余额不足提醒',
+      description: '当账户余额低于阈值时发送邮件提醒',
+      enabled: '启用余额不足提醒',
+      threshold: '自定义提醒阈值',
+      thresholdHint: '留空使用系统默认值',
+      thresholdPlaceholder: '输入金额',
+      systemDefault: '系统默认值',
+      extraEmails: '额外通知邮箱',
+      noExtraEmails: '暂无额外通知邮箱',
+      enterEmail: '输入邮箱地址',
+      addEmail: '添加邮箱',
+      emailPlaceholder: '输入邮箱地址',
+      sendCode: '发送验证码',
+      codeSent: '验证码已发送',
+      codeSentTo: '验证码已发送到 {email}',
+      enterCode: '输入验证码',
+      codePlaceholder: '6位验证码',
+      verify: '确认添加',
+      emailAdded: '邮箱已添加',
+      emailRemoved: '邮箱已移除',
+      verifySuccess: '邮箱添加成功',
+      removeEmail: '移除',
+      removeSuccess: '邮箱已移除',
     }
   },
 
@@ -2226,6 +2251,12 @@ export default {
       },
       quotaLimitAmount: '总限额',
       quotaLimitAmountHint: '累计消费上限，不会自动重置。',
+      quotaNotify: {
+        alert: '告警阈值',
+        enabled: '启用告警',
+        threshold: '告警金额',
+        thresholdPlaceholder: '输入告警金额',
+      },
       testConnection: '测试连接',
       reAuthorize: '重新授权',
       refreshToken: '刷新令牌',
@@ -4757,6 +4788,22 @@ export default {
         supportedTypesHint: '逗号分隔，如 alipay,wxpay',
         refundEnabled: '允许退款',
       },
+      balanceNotify: {
+        title: '余额不足提醒',
+        description: '当用户余额低于阈值时发送邮件提醒',
+        enabled: '启用余额不足提醒',
+        threshold: '默认提醒阈值',
+        thresholdHint: '用户未自定义时使用此值',
+        thresholdPlaceholder: '输入金额',
+      },
+      quotaNotify: {
+        title: '账号限额通知',
+        description: '当账号配额用量达到告警阈值时通知管理员',
+        emails: '通知邮箱',
+        emailsHint: '留空则不发送通知',
+        addEmail: '添加邮箱',
+        emailPlaceholder: '输入邮箱地址',
+      },
       smtp: {
         title: 'SMTP 设置',
         description: '配置用于发送验证码的邮件服务',