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;
feat: helper function in errors for checking postgres error types;
feat: sql query method for finding users by their email;
feat: registration begin/complete with checking existing username/email;
refactor: error handling in controller
