docs: add v2→v3 migration guide and document new v3 features

- v2→v3 breaking-change list: default mode flip, maxAttempts:5, deleteOn
  enum change (removed "failure"), schema migration for deadLetteredAt
- Document enqueueMany() with example
- Document per-job timeout (jobTimeout) with example
- Add "Dead-letter Queue & Monitoring" section: dead event, stats(),
  purge() usage
- Update options table: maxAttempts default, deleteOn clarification,
  new maxRetryDelay, staleTimeout, jobTimeout rows
- Update feature bullets to reflect new capabilities