init(api): add user authentication handler

also add error casting function to db.go

backend/db/db.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	"github.com/H1K0/SkazaNull/models"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
@@ -30,6 +31,14 @@ 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) {