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/user/user.go

@@ -43,6 +43,12 @@ const (
 	FieldTotpEnabled = "totp_enabled"
 	// FieldTotpEnabledAt holds the string denoting the totp_enabled_at field in the database.
 	FieldTotpEnabledAt = "totp_enabled_at"
+	// FieldBalanceNotifyEnabled holds the string denoting the balance_notify_enabled field in the database.
+	FieldBalanceNotifyEnabled = "balance_notify_enabled"
+	// FieldBalanceNotifyThreshold holds the string denoting the balance_notify_threshold field in the database.
+	FieldBalanceNotifyThreshold = "balance_notify_threshold"
+	// FieldBalanceNotifyExtraEmails holds the string denoting the balance_notify_extra_emails field in the database.
+	FieldBalanceNotifyExtraEmails = "balance_notify_extra_emails"
 	// EdgeAPIKeys holds the string denoting the api_keys edge name in mutations.
 	EdgeAPIKeys = "api_keys"
 	// EdgeRedeemCodes holds the string denoting the redeem_codes edge name in mutations.
@@ -161,6 +167,9 @@ var Columns = []string{
 	FieldTotpSecretEncrypted,
 	FieldTotpEnabled,
 	FieldTotpEnabledAt,
+	FieldBalanceNotifyEnabled,
+	FieldBalanceNotifyThreshold,
+	FieldBalanceNotifyExtraEmails,
 }
 
 var (
@@ -217,6 +226,10 @@ var (
 	DefaultNotes string
 	// DefaultTotpEnabled holds the default value on creation for the "totp_enabled" field.
 	DefaultTotpEnabled bool
+	// DefaultBalanceNotifyEnabled holds the default value on creation for the "balance_notify_enabled" field.
+	DefaultBalanceNotifyEnabled bool
+	// DefaultBalanceNotifyExtraEmails holds the default value on creation for the "balance_notify_extra_emails" field.
+	DefaultBalanceNotifyExtraEmails string
 )
 
 // OrderOption defines the ordering options for the User queries.
@@ -297,6 +310,21 @@ func ByTotpEnabledAt(opts ...sql.OrderTermOption) OrderOption {
 	return sql.OrderByField(FieldTotpEnabledAt, opts...).ToFunc()
 }
 
+// ByBalanceNotifyEnabled orders the results by the balance_notify_enabled field.
+func ByBalanceNotifyEnabled(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldBalanceNotifyEnabled, opts...).ToFunc()
+}
+
+// ByBalanceNotifyThreshold orders the results by the balance_notify_threshold field.
+func ByBalanceNotifyThreshold(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldBalanceNotifyThreshold, opts...).ToFunc()
+}
+
+// ByBalanceNotifyExtraEmails orders the results by the balance_notify_extra_emails field.
+func ByBalanceNotifyExtraEmails(opts ...sql.OrderTermOption) OrderOption {
+	return sql.OrderByField(FieldBalanceNotifyExtraEmails, opts...).ToFunc()
+}
+
 // ByAPIKeysCount orders the results by api_keys count.
 func ByAPIKeysCount(opts ...sql.OrderTermOption) OrderOption {
 	return func(s *sql.Selector) {