refactor: moved hashing logic into application layer for security;

fix: error handling in auth service for database;
refactor: removed redundant taken email check;
chore: removed todos that were completed/not needed;
fix: leaking transactions in complete registration and login on error;
refactor: got rid of txless requests during transactions;

backend/internal/errors/postgres.go

@@ -0,0 +1,17 @@
+package errors
+
+import (
+	"errors"
+
+	"github.com/jackc/pgx/v5/pgconn"
+)
+
+func GetPgError(err error) string {
+	var pgErr *pgconn.PgError
+	errors.As(err, &pgErr)
+	return pgErr.Code
+}
+
+func MatchPgError(err error, code string) bool {
+	return GetPgError(err) == code
+}