Use a Queue to Process One Request at a Time

My [app] has a critical action where [describe action, e.g. users withdraw from a shared pool / admins approve payouts / players trade items]. Database locks alone aren't enough because the logic spans multiple steps.

Set up a queue-based system that:
- Accepts incoming requests immediately and returns "processing" to the user
- Processes them one at a time (or in safe batches) in order
- Updates the shared resource only inside the queue worker, never from direct API calls
- Notifies the user when their request is complete or rejected
- Handles failures gracefully (retry or refund)

My stack: [your language and framework here]

I'm learning, so explain each part simply.