Update README with comprehensive v0.4.0 feature documentation

- Add job prioritization system documentation with priority levels and scheduling algorithms
- Add comprehensive cron scheduling section with timezone support and management API
- Add job timeout configuration and handling documentation
- Add statistics collection and dead job management sections
- Update database schema documentation with all new fields and indexes
- Update version references from 0.1 to 0.4.0 throughout
- Add documentation for new examples (cron_example.rs, priority_example.rs)
- Enhance feature list and code examples to showcase new capabilities

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: CodingAnarchy