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) {

backend/ent/user/where.go

@@ -125,6 +125,21 @@ func TotpEnabledAt(v time.Time) predicate.User {
 	return predicate.User(sql.FieldEQ(FieldTotpEnabledAt, v))
 }
 
+// BalanceNotifyEnabled applies equality check predicate on the "balance_notify_enabled" field. It's identical to BalanceNotifyEnabledEQ.
+func BalanceNotifyEnabled(v bool) predicate.User {
+	return predicate.User(sql.FieldEQ(FieldBalanceNotifyEnabled, v))
+}
+
+// BalanceNotifyThreshold applies equality check predicate on the "balance_notify_threshold" field. It's identical to BalanceNotifyThresholdEQ.
+func BalanceNotifyThreshold(v float64) predicate.User {
+	return predicate.User(sql.FieldEQ(FieldBalanceNotifyThreshold, v))
+}
+
+// BalanceNotifyExtraEmails applies equality check predicate on the "balance_notify_extra_emails" field. It's identical to BalanceNotifyExtraEmailsEQ.
+func BalanceNotifyExtraEmails(v string) predicate.User {
+	return predicate.User(sql.FieldEQ(FieldBalanceNotifyExtraEmails, v))
+}
+
 // CreatedAtEQ applies the EQ predicate on the "created_at" field.
 func CreatedAtEQ(v time.Time) predicate.User {
 	return predicate.User(sql.FieldEQ(FieldCreatedAt, v))
@@ -860,6 +875,131 @@ func TotpEnabledAtNotNil() predicate.User {
 	return predicate.User(sql.FieldNotNull(FieldTotpEnabledAt))
 }
 
+// BalanceNotifyEnabledEQ applies the EQ predicate on the "balance_notify_enabled" field.
+func BalanceNotifyEnabledEQ(v bool) predicate.User {
+	return predicate.User(sql.FieldEQ(FieldBalanceNotifyEnabled, v))
+}
+
+// BalanceNotifyEnabledNEQ applies the NEQ predicate on the "balance_notify_enabled" field.
+func BalanceNotifyEnabledNEQ(v bool) predicate.User {
+	return predicate.User(sql.FieldNEQ(FieldBalanceNotifyEnabled, v))
+}
+
+// BalanceNotifyThresholdEQ applies the EQ predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdEQ(v float64) predicate.User {
+	return predicate.User(sql.FieldEQ(FieldBalanceNotifyThreshold, v))
+}
+
+// BalanceNotifyThresholdNEQ applies the NEQ predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdNEQ(v float64) predicate.User {
+	return predicate.User(sql.FieldNEQ(FieldBalanceNotifyThreshold, v))
+}
+
+// BalanceNotifyThresholdIn applies the In predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdIn(vs ...float64) predicate.User {
+	return predicate.User(sql.FieldIn(FieldBalanceNotifyThreshold, vs...))
+}
+
+// BalanceNotifyThresholdNotIn applies the NotIn predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdNotIn(vs ...float64) predicate.User {
+	return predicate.User(sql.FieldNotIn(FieldBalanceNotifyThreshold, vs...))
+}
+
+// BalanceNotifyThresholdGT applies the GT predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdGT(v float64) predicate.User {
+	return predicate.User(sql.FieldGT(FieldBalanceNotifyThreshold, v))
+}
+
+// BalanceNotifyThresholdGTE applies the GTE predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdGTE(v float64) predicate.User {
+	return predicate.User(sql.FieldGTE(FieldBalanceNotifyThreshold, v))
+}
+
+// BalanceNotifyThresholdLT applies the LT predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdLT(v float64) predicate.User {
+	return predicate.User(sql.FieldLT(FieldBalanceNotifyThreshold, v))
+}
+
+// BalanceNotifyThresholdLTE applies the LTE predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdLTE(v float64) predicate.User {
+	return predicate.User(sql.FieldLTE(FieldBalanceNotifyThreshold, v))
+}
+
+// BalanceNotifyThresholdIsNil applies the IsNil predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdIsNil() predicate.User {
+	return predicate.User(sql.FieldIsNull(FieldBalanceNotifyThreshold))
+}
+
+// BalanceNotifyThresholdNotNil applies the NotNil predicate on the "balance_notify_threshold" field.
+func BalanceNotifyThresholdNotNil() predicate.User {
+	return predicate.User(sql.FieldNotNull(FieldBalanceNotifyThreshold))
+}
+
+// BalanceNotifyExtraEmailsEQ applies the EQ predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsEQ(v string) predicate.User {
+	return predicate.User(sql.FieldEQ(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsNEQ applies the NEQ predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsNEQ(v string) predicate.User {
+	return predicate.User(sql.FieldNEQ(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsIn applies the In predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsIn(vs ...string) predicate.User {
+	return predicate.User(sql.FieldIn(FieldBalanceNotifyExtraEmails, vs...))
+}
+
+// BalanceNotifyExtraEmailsNotIn applies the NotIn predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsNotIn(vs ...string) predicate.User {
+	return predicate.User(sql.FieldNotIn(FieldBalanceNotifyExtraEmails, vs...))
+}
+
+// BalanceNotifyExtraEmailsGT applies the GT predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsGT(v string) predicate.User {
+	return predicate.User(sql.FieldGT(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsGTE applies the GTE predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsGTE(v string) predicate.User {
+	return predicate.User(sql.FieldGTE(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsLT applies the LT predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsLT(v string) predicate.User {
+	return predicate.User(sql.FieldLT(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsLTE applies the LTE predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsLTE(v string) predicate.User {
+	return predicate.User(sql.FieldLTE(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsContains applies the Contains predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsContains(v string) predicate.User {
+	return predicate.User(sql.FieldContains(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsHasPrefix applies the HasPrefix predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsHasPrefix(v string) predicate.User {
+	return predicate.User(sql.FieldHasPrefix(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsHasSuffix applies the HasSuffix predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsHasSuffix(v string) predicate.User {
+	return predicate.User(sql.FieldHasSuffix(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsEqualFold applies the EqualFold predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsEqualFold(v string) predicate.User {
+	return predicate.User(sql.FieldEqualFold(FieldBalanceNotifyExtraEmails, v))
+}
+
+// BalanceNotifyExtraEmailsContainsFold applies the ContainsFold predicate on the "balance_notify_extra_emails" field.
+func BalanceNotifyExtraEmailsContainsFold(v string) predicate.User {
+	return predicate.User(sql.FieldContainsFold(FieldBalanceNotifyExtraEmails, v))
+}
+
 // HasAPIKeys applies the HasEdge predicate on the "api_keys" edge.
 func HasAPIKeys() predicate.User {
 	return predicate.User(func(s *sql.Selector) {