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/api/admin/settings.ts

@@ -4,7 +4,7 @@
  */
 
 import { apiClient } from '../client'
-import type { CustomMenuItem, CustomEndpoint } from '@/types'
+import type { CustomMenuItem, CustomEndpoint, NotifyEmailEntry } from '@/types'
 
 export interface DefaultSubscriptionSetting {
   group_id: number
@@ -139,7 +139,7 @@ export interface SystemSettings {
   balance_low_notify_enabled: boolean
   balance_low_notify_threshold: number
   account_quota_notify_enabled: boolean
-  account_quota_notify_emails: string[]
+  account_quota_notify_emails: NotifyEmailEntry[]
 }
 
 export interface UpdateSettingsRequest {
@@ -243,7 +243,7 @@ export interface UpdateSettingsRequest {
   balance_low_notify_enabled?: boolean
   balance_low_notify_threshold?: number
   account_quota_notify_enabled?: boolean
-  account_quota_notify_emails?: string[]
+  account_quota_notify_emails?: NotifyEmailEntry[]
 }
 
 /**