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.go

@@ -45,6 +45,12 @@ type User struct {
 	TotpEnabled bool `json:"totp_enabled,omitempty"`
 	// TotpEnabledAt holds the value of the "totp_enabled_at" field.
 	TotpEnabledAt *time.Time `json:"totp_enabled_at,omitempty"`
+	// BalanceNotifyEnabled holds the value of the "balance_notify_enabled" field.
+	BalanceNotifyEnabled bool `json:"balance_notify_enabled,omitempty"`
+	// BalanceNotifyThreshold holds the value of the "balance_notify_threshold" field.
+	BalanceNotifyThreshold *float64 `json:"balance_notify_threshold,omitempty"`
+	// BalanceNotifyExtraEmails holds the value of the "balance_notify_extra_emails" field.
+	BalanceNotifyExtraEmails string `json:"balance_notify_extra_emails,omitempty"`
 	// Edges holds the relations/edges for other nodes in the graph.
 	// The values are being populated by the UserQuery when eager-loading is set.
 	Edges        UserEdges `json:"edges"`
@@ -184,13 +190,13 @@ func (*User) scanValues(columns []string) ([]any, error) {
 	values := make([]any, len(columns))
 	for i := range columns {
 		switch columns[i] {
-		case user.FieldTotpEnabled:
+		case user.FieldTotpEnabled, user.FieldBalanceNotifyEnabled:
 			values[i] = new(sql.NullBool)
-		case user.FieldBalance:
+		case user.FieldBalance, user.FieldBalanceNotifyThreshold:
 			values[i] = new(sql.NullFloat64)
 		case user.FieldID, user.FieldConcurrency:
 			values[i] = new(sql.NullInt64)
-		case user.FieldEmail, user.FieldPasswordHash, user.FieldRole, user.FieldStatus, user.FieldUsername, user.FieldNotes, user.FieldTotpSecretEncrypted:
+		case user.FieldEmail, user.FieldPasswordHash, user.FieldRole, user.FieldStatus, user.FieldUsername, user.FieldNotes, user.FieldTotpSecretEncrypted, user.FieldBalanceNotifyExtraEmails:
 			values[i] = new(sql.NullString)
 		case user.FieldCreatedAt, user.FieldUpdatedAt, user.FieldDeletedAt, user.FieldTotpEnabledAt:
 			values[i] = new(sql.NullTime)
@@ -302,6 +308,25 @@ func (_m *User) assignValues(columns []string, values []any) error {
 				_m.TotpEnabledAt = new(time.Time)
 				*_m.TotpEnabledAt = value.Time
 			}
+		case user.FieldBalanceNotifyEnabled:
+			if value, ok := values[i].(*sql.NullBool); !ok {
+				return fmt.Errorf("unexpected type %T for field balance_notify_enabled", values[i])
+			} else if value.Valid {
+				_m.BalanceNotifyEnabled = value.Bool
+			}
+		case user.FieldBalanceNotifyThreshold:
+			if value, ok := values[i].(*sql.NullFloat64); !ok {
+				return fmt.Errorf("unexpected type %T for field balance_notify_threshold", values[i])
+			} else if value.Valid {
+				_m.BalanceNotifyThreshold = new(float64)
+				*_m.BalanceNotifyThreshold = value.Float64
+			}
+		case user.FieldBalanceNotifyExtraEmails:
+			if value, ok := values[i].(*sql.NullString); !ok {
+				return fmt.Errorf("unexpected type %T for field balance_notify_extra_emails", values[i])
+			} else if value.Valid {
+				_m.BalanceNotifyExtraEmails = value.String
+			}
 		default:
 			_m.selectValues.Set(columns[i], values[i])
 		}
@@ -440,6 +465,17 @@ func (_m *User) String() string {
 		builder.WriteString("totp_enabled_at=")
 		builder.WriteString(v.Format(time.ANSIC))
 	}
+	builder.WriteString(", ")
+	builder.WriteString("balance_notify_enabled=")
+	builder.WriteString(fmt.Sprintf("%v", _m.BalanceNotifyEnabled))
+	builder.WriteString(", ")
+	if v := _m.BalanceNotifyThreshold; v != nil {
+		builder.WriteString("balance_notify_threshold=")
+		builder.WriteString(fmt.Sprintf("%v", *v))
+	}
+	builder.WriteString(", ")
+	builder.WriteString("balance_notify_extra_emails=")
+	builder.WriteString(_m.BalanceNotifyExtraEmails)
 	builder.WriteByte(')')
 	return builder.String()
 }