feat(notify): add balance low & account quota notification system

- User balance low notification: email alert when balance drops below
  configurable threshold (user email + verified extra emails)
- Account quota notification: broadcast email to admin-configured
  recipients when daily/weekly/total quota usage exceeds alert threshold
- Admin settings: global enable/disable, default threshold, quota
  notification email list (Email Settings tab)
- User profile: enable/disable, custom threshold, add/remove extra
  notification emails with verification code flow
- Account quota: per-dimension alert toggle and threshold in quota
  control card
- Trigger logic: first-crossing only (old >= threshold && new < threshold
  for balance; old < threshold && new >= threshold for quota), naturally
  prevents duplicate notifications without Redis dedup

backend/ent/runtime/runtime.go

@@ -1293,6 +1293,14 @@ func init() {
 	userDescTotpEnabled := userFields[9].Descriptor()
 	// user.DefaultTotpEnabled holds the default value on creation for the totp_enabled field.
 	user.DefaultTotpEnabled = userDescTotpEnabled.Default.(bool)
+	// userDescBalanceNotifyEnabled is the schema descriptor for balance_notify_enabled field.
+	userDescBalanceNotifyEnabled := userFields[11].Descriptor()
+	// user.DefaultBalanceNotifyEnabled holds the default value on creation for the balance_notify_enabled field.
+	user.DefaultBalanceNotifyEnabled = userDescBalanceNotifyEnabled.Default.(bool)
+	// userDescBalanceNotifyExtraEmails is the schema descriptor for balance_notify_extra_emails field.
+	userDescBalanceNotifyExtraEmails := userFields[13].Descriptor()
+	// user.DefaultBalanceNotifyExtraEmails holds the default value on creation for the balance_notify_extra_emails field.
+	user.DefaultBalanceNotifyExtraEmails = userDescBalanceNotifyExtraEmails.Default.(string)
 	userallowedgroupFields := schema.UserAllowedGroup{}.Fields()
 	_ = userallowedgroupFields
 	// userallowedgroupDescCreatedAt is the schema descriptor for created_at field.