Async Patterns

Summary

The practical patterns for structuring async backend code cleanly — sequential vs parallel awaits, error propagation, and avoiding callback hell in real route handlers.

• 1Refactor a route handler with multiple sequential awaits into parallel ones where possible.
• 2Write a reusable `asyncHandler` wrapper to catch errors in Express route handlers.
• 3Practice chaining dependent async calls (e.g. create user, then send welcome email) correctly.