Add comprehensive job batching and bulk operations support

This implementation adds high-performance batch processing capabilities to Hammerwork, addressing the top priority roadmap item with significant performance improvements for high-throughput scenarios.

## Key Features Added

### Core Batch Infrastructure
- JobBatch struct with configurable batch settings and failure handling
- PartialFailureMode enum (ContinueOnError, FailFast, CollectErrors)
- BatchStatus enum for tracking batch progress
- BatchResult struct for comprehensive batch monitoring

### Database Backend Implementation
- PostgreSQL: Optimized bulk INSERT using UNNEST arrays for maximum performance
- MySQL: Multi-row VALUES syntax with chunking for MySQL compatibility
- New hammerwork_batches table for batch metadata tracking
- batch_id column added to hammerwork_jobs table

### API Enhancements
- DatabaseQueue trait extended with batch operations:
  - enqueue_batch() for bulk job insertion
  - get_batch_status() for progress monitoring
  - get_batch_jobs() for batch job retrieval
  - delete_batch() for cleanup operations

### Comprehensive Testing
- Full test suite in tests/batch_tests.rs covering all batch functionality
- Database-agnostic tests for JobBatch struct behavior
- PostgreSQL and MySQL specific integration tests
- Edge case testing for validation and error handling

### Documentation and Examples
- Complete batch_example.rs demonstrating real-world usage patterns
- Updated README.md highlighting batch operation capabilities
- Comprehensive inline documentation with doctests

## Technical Implementation

The batch system is designed for optimal performance:
- PostgreSQL uses UNNEST for true bulk operations
- MySQL uses prepared statements with chunking
- Atomic database transactions ensure consistency
- Configurable batch sizes prevent memory issues
- Extensive validation prevents common errors

## Performance Impact

This feature enables significant performance improvements:
- Single database transaction for entire batch
- Reduced network round-trips for large job sets
- Optimized database queries with bulk operations
- Configurable chunking for memory efficiency

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

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

Author: CodingAnarchy

README.md

@@ -6,6 +6,7 @@ A high-performance, database-driven job queue for Rust with comprehensive featur
 
 - **Multi-database support**: PostgreSQL and MySQL backends
 - **Job prioritization**: Five priority levels with weighted and strict scheduling algorithms
+- **Batch operations**: High-performance bulk job enqueuing for improved throughput
 - **Cron scheduling**: Full cron expression support with timezone awareness
 - **Rate limiting**: Token bucket rate limiting with configurable burst limits
 - **Monitoring**: Prometheus metrics and advanced alerting (enabled by default)