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

backend/internal/service/user.go

@@ -34,7 +34,7 @@ type User struct {
 	BalanceNotifyEnabled       bool
 	BalanceNotifyThresholdType string   // "fixed" (default) | "percentage"
 	BalanceNotifyThreshold     *float64
-	BalanceNotifyExtraEmails   []string
+	BalanceNotifyExtraEmails   []NotifyEmailEntry
 	TotalRecharged             float64
 
 	APIKeys       []APIKey