docs: add asynchronous jobs section to webapp README

[konokenj]

Summary

Added documentation for creating asynchronous jobs in the webapp README.

Changes

• Added "Asynchronous Jobs" section explaining how to create Lambda-based background jobs
• Documented the build process using job.Dockerfile with esbuild
• Provided two patterns: single-file jobs and jobs with subdirectories
• Included practical examples based on existing code (async-job-runner.ts and migration-runner.ts)

Key Points

• All jobs in src/jobs/*.ts are bundled into separate Lambda handlers
• Simple jobs use a single file; complex jobs use subdirectories for organization
• No individual Dockerfiles needed - shared job.Dockerfile handles all jobs
• Jobs must be configured in CDK stack for deployment

The documentation is concise, accurate, and follows the existing project patterns.

[tmokmss]

Thanks!