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/types/index.ts

@@ -22,6 +22,16 @@ export interface FetchOptions {
   signal?: AbortSignal
 }
 
+// ==================== Notification Types ====================
+
+/** Notification email entry with enable/disable and verification state.
+ *  email="" is a placeholder for the primary email (user's registration email or admin email). */
+export interface NotifyEmailEntry {
+  email: string
+  disabled: boolean
+  verified: boolean
+}
+
 // ==================== User & Auth Types ====================
 
 export interface User {
@@ -35,7 +45,7 @@ export interface User {
   allowed_groups: number[] | null // Allowed group IDs (null = all non-exclusive groups)
   balance_notify_enabled: boolean
   balance_notify_threshold: number | null
-  balance_notify_extra_emails: string[]
+  balance_notify_extra_emails: NotifyEmailEntry[]
   subscriptions?: UserSubscription[] // User's active subscriptions
   created_at: string
   updated_at: string