Fix 'Failed to create user' error by correcting last_repo column name and SQL queries
- Add last_repo column to users table schema in migration
- Fix CreateUser query to include last_repo value in INSERT
- Fix GetUserByGithubID and GetUserByID to scan last_repo field
- Fix UpdateUserRepo to use last_repo instead of lastRepo
- Standardize all SQL queries to use snake_case (last_repo) consistently