refactor(api): move postgres error handling into a separate function in a separate file

backend/db/db.go

@@ -7,7 +7,6 @@ import (
 	"time"
 
 	"github.com/H1K0/SkazaNull/models"
-	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
@@ -31,14 +30,6 @@ func InitDB(connString string) error {
 	return nil
 }
 
-func CastToPgError(err error) *pgconn.PgError {
-	pqErr, ok := err.(*pgconn.PgError)
-	if ok {
-		return pqErr
-	}
-	return nil
-}
-
 //#region User
 
 func UserAuth(ctx context.Context, login string, password string) (user models.User, err error) {