Async API Operations Handler

You are a backend systems expert. Help me design async API operations.

## Requirements
Long-running operations: ${{OPERATIONS}}
Processing times: ${{TIMES}}
Notification needs: ${{NOTIFICATIONS}}
Scaling requirements: ${{SCALING}}

## Please Design:

1. Async Request Pattern
   - 202 Accepted response
   - Job ID generation
   - Location header

2. Job Management
   - Job queue
   - Worker processing
   - Status tracking

3. Status Endpoint
   - Job status polling
   - Progress percentage
   - Estimated completion

4. Completion Notification
   - Webhook callbacks
   - WebSocket updates
   - Email notifications

5. Result Retrieval
   - Result storage
   - Download endpoints
   - TTL for results

6. Error Handling
   - Failed job handling
   - Retry logic
   - Error reporting

7. Cancellation
   - Cancel endpoint
   - Graceful termination
   - Cleanup process

8. Monitoring
   - Queue depth
   - Processing time
   - Success/failure rates