feat(notify): convert email lists to NotifyEmailEntry struct with toggle support

- Change balance_notify_extra_emails and account_quota_notify_emails
  from []string to []NotifyEmailEntry{email, disabled, verified}
- Add per-email enable/disable toggle for both user and admin notifications
- Add PUT /user/notify-email/toggle API endpoint
- Fix critical bug: API key auth cache snapshot missing balance notify
  fields (Email, Username, BalanceNotifyEnabled, etc.), causing
  notifications to never fire on cached request paths
- Bump cache snapshot version 3→4 to invalidate stale entries
- Add SQL migration 104 to convert old format data
- Backward compatible: parseNotifyEmails auto-detects old/new format
- User balance notify: max 3 emails (primary + 2 extra)
- Admin quota notify: unlimited emails, each with toggle

frontend/src/i18n/locales/en.ts

@@ -930,6 +930,8 @@ export default {
       removeEmail: 'Remove',
       removeSuccess: 'Email removed',
       emailDuplicate: 'This email already exists',
+      maxEmailsReached: 'Maximum number of notification emails reached',
+      unverified: 'Unverified',
     }
   },
 

frontend/src/i18n/locales/zh.ts

@@ -934,6 +934,8 @@ export default {
       removeEmail: '移除',
       removeSuccess: '邮箱已移除',
       emailDuplicate: '该邮箱已存在',
+      maxEmailsReached: '已达到通知邮箱数量上限',
+      unverified: '未验证',
     }
   },