feat: update callback handler and repository for user

backend/internal/storage/migrations.go

@@ -11,7 +11,7 @@ CREATE TABLE IF NOT EXISTS posts(
 CREATE TABLE IF NOT EXISTS users(
 	id INTEGER PRIMARY KEY AUTOINCREMENT,
 	email TEXT,
-	github_id TEXT,
+	github_id INTEGER,
 	github_login TEXT,
 	avatar_url TEXT
 );