refactor/fix: now using pgx errors for postgres error checking instead of trying to look up the error code;

feat: implemented working custom validation;
fix: authservice begin/complete registration

backend/internal/errors/postgres.go

@@ -1,34 +0,0 @@
-// Copyright (c) 2025 Nikolai Papin
-//
-// This file is part of Easywish
-//
-// This program is free software: you can redistribute it and/or modify
-// it under the terms of the GNU General Public License as published by
-// the Free Software Foundation, either version 3 of the License, or
-// (at your option) any later version.
-//
-// This program is distributed in the hope that it will be useful,
-// but WITHOUT ANY WARRANTY; without even the implied warranty of
-// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
-// the GNU General Public License for more details.
-//
-// You should have received a copy of the GNU General Public License
-// along with this program.  If not, see <https://www.gnu.org/licenses/>.
-
-package errors
-
-import (
-	"errors"
-
-	"github.com/jackc/pgx/v5/pgconn"
-)
-
-func IsPgErr(err error, code string) bool {
-	var pgErr *pgconn.PgError
-	if errors.As(err, &pgErr) {
-		if pgErr.Code == code {
-			return true
-		}
-	}
-	return false
-}