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/handlers/portfolio/images.go

@@ -24,6 +24,7 @@ import (
 	"time"
 
 	"github.com/go-chi/chi/v5"
+	"github.com/jackc/pgx/v5"
 	"github.com/kovidgoyal/imaging"
 )
 
@@ -903,7 +904,7 @@ func UploadImage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	defer func() {
-		if err := tx.Rollback(r.Context()); err != nil && err.Error() != "tx is closed" {
+		if err := tx.Rollback(r.Context()); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
 			slog.Error("failed to rollback transaction", "err", err)
 		}
 	}()
@@ -1038,7 +1039,7 @@ func DeleteImage(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	defer func() {
-		if err := tx.Rollback(r.Context()); err != nil && err.Error() != "tx is closed" {
+		if err := tx.Rollback(r.Context()); err != nil && !errors.Is(err, pgx.ErrTxClosed) {
 			slog.Error("failed to rollback transaction", "err", err)
 		}
 	}()