fix: replace err.Error() string match with errors.Is(err, pgx.ErrTxClosed)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/auth/jwt.go

@@ -49,7 +49,7 @@ func RevokeJTI(ctx context.Context, jti string, expiresAt time.Time) error {
 		return fmt.Errorf("revoke JTI: begin transaction: %w", err)
 	}
 	defer func() {
-		if err := tx.Rollback(ctx); err != nil && err.Error() != "tx is closed" {
+		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
 			slog.Error("failed to rollback transaction", "err", err)
 		}
 	}()
@@ -98,10 +98,10 @@ func CleanupRevokedJTIs(ctx context.Context) (int, error) {
 		return 0, err
 	}
 	defer func() {
-    if err := tx.Rollback(ctx); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	tag, err := tx.Exec(ctx,
 		`DELETE FROM revoked_jtis WHERE expires_at < NOW()`)
@@ -208,10 +208,10 @@ func GenerateRefreshToken(ctx context.Context, userID string, role string) (stri
 		return "", fmt.Errorf("failed to begin transaction: %w", err)
 	}
 	defer func() {
-    if err := tx.Rollback(ctx); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	var tokenID int64
 	err = tx.QueryRow(ctx, query, userID, token, role).Scan(&tokenID)
@@ -241,10 +241,10 @@ func VerifyRefreshToken(ctx context.Context, tokenString string) (userID string,
 		return "", "", fmt.Errorf("failed to begin transaction: %w", err)
 	}
 	defer func() {
-    if err := tx.Rollback(ctx); err != nil && err.Error() != "tx is closed" {
-        slog.Error("failed to rollback transaction", "err", err)
-    }
-}()
+		if err := tx.Rollback(ctx); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
+			slog.Error("failed to rollback transaction", "err", err)
+		}
+	}()
 
 	err = tx.QueryRow(ctx, query, tokenString).Scan(&userID, &role)
 	if err != nil {