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_update.go

@@ -243,6 +243,61 @@ func (_u *UserUpdate) ClearTotpEnabledAt() *UserUpdate {
 	return _u
 }
 
+// SetBalanceNotifyEnabled sets the "balance_notify_enabled" field.
+func (_u *UserUpdate) SetBalanceNotifyEnabled(v bool) *UserUpdate {
+	_u.mutation.SetBalanceNotifyEnabled(v)
+	return _u
+}
+
+// SetNillableBalanceNotifyEnabled sets the "balance_notify_enabled" field if the given value is not nil.
+func (_u *UserUpdate) SetNillableBalanceNotifyEnabled(v *bool) *UserUpdate {
+	if v != nil {
+		_u.SetBalanceNotifyEnabled(*v)
+	}
+	return _u
+}
+
+// SetBalanceNotifyThreshold sets the "balance_notify_threshold" field.
+func (_u *UserUpdate) SetBalanceNotifyThreshold(v float64) *UserUpdate {
+	_u.mutation.ResetBalanceNotifyThreshold()
+	_u.mutation.SetBalanceNotifyThreshold(v)
+	return _u
+}
+
+// SetNillableBalanceNotifyThreshold sets the "balance_notify_threshold" field if the given value is not nil.
+func (_u *UserUpdate) SetNillableBalanceNotifyThreshold(v *float64) *UserUpdate {
+	if v != nil {
+		_u.SetBalanceNotifyThreshold(*v)
+	}
+	return _u
+}
+
+// AddBalanceNotifyThreshold adds value to the "balance_notify_threshold" field.
+func (_u *UserUpdate) AddBalanceNotifyThreshold(v float64) *UserUpdate {
+	_u.mutation.AddBalanceNotifyThreshold(v)
+	return _u
+}
+
+// ClearBalanceNotifyThreshold clears the value of the "balance_notify_threshold" field.
+func (_u *UserUpdate) ClearBalanceNotifyThreshold() *UserUpdate {
+	_u.mutation.ClearBalanceNotifyThreshold()
+	return _u
+}
+
+// SetBalanceNotifyExtraEmails sets the "balance_notify_extra_emails" field.
+func (_u *UserUpdate) SetBalanceNotifyExtraEmails(v string) *UserUpdate {
+	_u.mutation.SetBalanceNotifyExtraEmails(v)
+	return _u
+}
+
+// SetNillableBalanceNotifyExtraEmails sets the "balance_notify_extra_emails" field if the given value is not nil.
+func (_u *UserUpdate) SetNillableBalanceNotifyExtraEmails(v *string) *UserUpdate {
+	if v != nil {
+		_u.SetBalanceNotifyExtraEmails(*v)
+	}
+	return _u
+}
+
 // AddAPIKeyIDs adds the "api_keys" edge to the APIKey entity by IDs.
 func (_u *UserUpdate) AddAPIKeyIDs(ids ...int64) *UserUpdate {
 	_u.mutation.AddAPIKeyIDs(ids...)
@@ -746,6 +801,21 @@ func (_u *UserUpdate) sqlSave(ctx context.Context) (_node int, err error) {
 	if _u.mutation.TotpEnabledAtCleared() {
 		_spec.ClearField(user.FieldTotpEnabledAt, field.TypeTime)
 	}
+	if value, ok := _u.mutation.BalanceNotifyEnabled(); ok {
+		_spec.SetField(user.FieldBalanceNotifyEnabled, field.TypeBool, value)
+	}
+	if value, ok := _u.mutation.BalanceNotifyThreshold(); ok {
+		_spec.SetField(user.FieldBalanceNotifyThreshold, field.TypeFloat64, value)
+	}
+	if value, ok := _u.mutation.AddedBalanceNotifyThreshold(); ok {
+		_spec.AddField(user.FieldBalanceNotifyThreshold, field.TypeFloat64, value)
+	}
+	if _u.mutation.BalanceNotifyThresholdCleared() {
+		_spec.ClearField(user.FieldBalanceNotifyThreshold, field.TypeFloat64)
+	}
+	if value, ok := _u.mutation.BalanceNotifyExtraEmails(); ok {
+		_spec.SetField(user.FieldBalanceNotifyExtraEmails, field.TypeString, value)
+	}
 	if _u.mutation.APIKeysCleared() {
 		edge := &sqlgraph.EdgeSpec{
 			Rel:     sqlgraph.O2M,
@@ -1434,6 +1504,61 @@ func (_u *UserUpdateOne) ClearTotpEnabledAt() *UserUpdateOne {
 	return _u
 }
 
+// SetBalanceNotifyEnabled sets the "balance_notify_enabled" field.
+func (_u *UserUpdateOne) SetBalanceNotifyEnabled(v bool) *UserUpdateOne {
+	_u.mutation.SetBalanceNotifyEnabled(v)
+	return _u
+}
+
+// SetNillableBalanceNotifyEnabled sets the "balance_notify_enabled" field if the given value is not nil.
+func (_u *UserUpdateOne) SetNillableBalanceNotifyEnabled(v *bool) *UserUpdateOne {
+	if v != nil {
+		_u.SetBalanceNotifyEnabled(*v)
+	}
+	return _u
+}
+
+// SetBalanceNotifyThreshold sets the "balance_notify_threshold" field.
+func (_u *UserUpdateOne) SetBalanceNotifyThreshold(v float64) *UserUpdateOne {
+	_u.mutation.ResetBalanceNotifyThreshold()
+	_u.mutation.SetBalanceNotifyThreshold(v)
+	return _u
+}
+
+// SetNillableBalanceNotifyThreshold sets the "balance_notify_threshold" field if the given value is not nil.
+func (_u *UserUpdateOne) SetNillableBalanceNotifyThreshold(v *float64) *UserUpdateOne {
+	if v != nil {
+		_u.SetBalanceNotifyThreshold(*v)
+	}
+	return _u
+}
+
+// AddBalanceNotifyThreshold adds value to the "balance_notify_threshold" field.
+func (_u *UserUpdateOne) AddBalanceNotifyThreshold(v float64) *UserUpdateOne {
+	_u.mutation.AddBalanceNotifyThreshold(v)
+	return _u
+}
+
+// ClearBalanceNotifyThreshold clears the value of the "balance_notify_threshold" field.
+func (_u *UserUpdateOne) ClearBalanceNotifyThreshold() *UserUpdateOne {
+	_u.mutation.ClearBalanceNotifyThreshold()
+	return _u
+}
+
+// SetBalanceNotifyExtraEmails sets the "balance_notify_extra_emails" field.
+func (_u *UserUpdateOne) SetBalanceNotifyExtraEmails(v string) *UserUpdateOne {
+	_u.mutation.SetBalanceNotifyExtraEmails(v)
+	return _u
+}
+
+// SetNillableBalanceNotifyExtraEmails sets the "balance_notify_extra_emails" field if the given value is not nil.
+func (_u *UserUpdateOne) SetNillableBalanceNotifyExtraEmails(v *string) *UserUpdateOne {
+	if v != nil {
+		_u.SetBalanceNotifyExtraEmails(*v)
+	}
+	return _u
+}
+
 // AddAPIKeyIDs adds the "api_keys" edge to the APIKey entity by IDs.
 func (_u *UserUpdateOne) AddAPIKeyIDs(ids ...int64) *UserUpdateOne {
 	_u.mutation.AddAPIKeyIDs(ids...)
@@ -1967,6 +2092,21 @@ func (_u *UserUpdateOne) sqlSave(ctx context.Context) (_node *User, err error) {
 	if _u.mutation.TotpEnabledAtCleared() {
 		_spec.ClearField(user.FieldTotpEnabledAt, field.TypeTime)
 	}
+	if value, ok := _u.mutation.BalanceNotifyEnabled(); ok {
+		_spec.SetField(user.FieldBalanceNotifyEnabled, field.TypeBool, value)
+	}
+	if value, ok := _u.mutation.BalanceNotifyThreshold(); ok {
+		_spec.SetField(user.FieldBalanceNotifyThreshold, field.TypeFloat64, value)
+	}
+	if value, ok := _u.mutation.AddedBalanceNotifyThreshold(); ok {
+		_spec.AddField(user.FieldBalanceNotifyThreshold, field.TypeFloat64, value)
+	}
+	if _u.mutation.BalanceNotifyThresholdCleared() {
+		_spec.ClearField(user.FieldBalanceNotifyThreshold, field.TypeFloat64)
+	}
+	if value, ok := _u.mutation.BalanceNotifyExtraEmails(); ok {
+		_spec.SetField(user.FieldBalanceNotifyExtraEmails, field.TypeString, value)
+	}
 	if _u.mutation.APIKeysCleared() {
 		edge := &sqlgraph.EdgeSpec{
 			Rel:     sqlgraph.O2M,