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/api_key_auth_cache.go

@@ -34,6 +34,14 @@ type APIKeyAuthUserSnapshot struct {
 	Role        string  `json:"role"`
 	Balance     float64 `json:"balance"`
 	Concurrency int     `json:"concurrency"`
+
+	// Balance notification fields (required for CheckBalanceAfterDeduction)
+	Email                      string             `json:"email"`
+	Username                   string             `json:"username"`
+	BalanceNotifyEnabled       bool               `json:"balance_notify_enabled"`
+	BalanceNotifyThresholdType string             `json:"balance_notify_threshold_type"`
+	BalanceNotifyThreshold     *float64           `json:"balance_notify_threshold,omitempty"`
+	BalanceNotifyExtraEmails   []NotifyEmailEntry `json:"balance_notify_extra_emails,omitempty"`
 }
 
 // APIKeyAuthGroupSnapshot 分组快照