Add comprehensive CHANGELOG.md documenting all releases

Documents the complete version history of Hammerwork:
- v0.2.1: Remove "full" feature for cleaner database backend selection
- v0.2.0: Major release with statistics tracking and dead job management
- v0.1.0: Initial release with core job queue functionality

The changelog follows Keep a Changelog format and provides detailed
information about features, changes, and technical improvements across
all versions for better project maintenance and user understanding.

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

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

Author: CodingAnarchy

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [0.2.1] - 2025-06-25
+
+### Removed
+- Removed the `full` feature flag that enabled both PostgreSQL and MySQL simultaneously
+  - Users typically choose one database backend per application
+  - Simplifies the feature set and reduces unnecessary dependencies
+  - Available features are now: `postgres`, `mysql`
+
+## [0.2.0] - 2025-06-25
+
+### Added
+- **Comprehensive Statistics Tracking**
+  - `JobStatistics` struct with detailed metrics (throughput, processing times, error rates)
+  - `StatisticsCollector` trait for pluggable statistics backends
+  - `InMemoryStatsCollector` with time-windowed data collection and configurable cleanup
+  - `QueueStats` for queue-specific insights
+  - `DeadJobSummary` for dead job analysis with error patterns
+  - `JobEvent` system for tracking job processing lifecycle events
+  - Integration with `Worker` and `WorkerPool` for automatic statistics collection
+
+- **Dead Job Management**
+  - Enhanced `Job` struct with `failed_at` field and `Dead` status
+  - Dead job utility methods: `is_dead()`, `has_exhausted_retries()`, `age()`, `processing_duration()`
+  - Database operations for dead job management:
+    - `mark_job_dead()` - Mark jobs as permanently failed
+    - `get_dead_jobs()` - Retrieve dead jobs with pagination
+    - `get_dead_jobs_by_queue()` - Queue-specific dead job retrieval
+    - `retry_dead_job()` - Reset dead jobs for retry
+    - `purge_dead_jobs()` - Clean up old dead jobs
+    - `get_dead_job_summary()` - Summary statistics with error patterns
+
+- **Database Enhancements**
+  - Extended `DatabaseQueue` trait with 15 new methods for statistics and dead job management
+  - Enhanced database schemas with `failed_at` column and performance indexes
+  - Complete feature parity between PostgreSQL and MySQL implementations
+  - Optimized queries with proper indexing for performance
+
+- **Testing & Examples**
+  - 32 comprehensive unit tests with full coverage of new features
+  - Updated examples demonstrating statistics collection and dead job management
+  - Enhanced integration tests with 4 new test scenarios:
+    - Dead job management lifecycle
+    - Statistics collection functionality
+    - Database queue statistics
+    - Error frequency analysis
+
+### Changed
+- Updated package description to highlight new statistics and dead job management features
+- Enhanced API with backward-compatible design
+
+## [0.1.0] - 2025-06-25
+
+### Added
+- **Core Job Queue Infrastructure**
+  - `Job` struct with UUID, payload, status, retry logic, and scheduling
+  - Job statuses: `Pending`, `Running`, `Completed`, `Failed`, `Retrying`
+  - Support for delayed job execution with configurable retry attempts
+  - `JobQueue<DB>` generic struct with database connection pooling
+
+- **Worker System**
+  - `Worker<DB>` for processing jobs from specific queues
+  - Configurable polling intervals, retry delays, and maximum retries
+  - `WorkerPool<DB>` for managing multiple workers with graceful shutdown
+  - Async channels for coordinated shutdown signaling
+
+- **Database Support**
+  - `DatabaseQueue` trait defining interface for database operations
+  - Full PostgreSQL implementation with `FOR UPDATE SKIP LOCKED` for efficient job polling
+  - Full MySQL implementation with transaction-based locking
+  - Database-agnostic design using SQLx for compile-time query verification
+
+- **Error Handling & Observability**
+  - `HammerworkError` enum with structured error handling using `thiserror`
+  - Comprehensive error wrapping for SQLx, serialization, and custom errors
+  - Integrated tracing support for observability
+
+- **Database Schema**
+  - Optimized `hammerwork_jobs` table design for both PostgreSQL and MySQL
+  - Proper indexing for efficient queue polling and job management
+  - JSON/JSONB payload support with metadata tracking
+
+- **Testing & Integration**
+  - Comprehensive Docker-based integration testing infrastructure
+  - Support for both PostgreSQL and MySQL in CI/CD pipelines
+  - Shared test scenarios for database-agnostic testing
+  - Complete test coverage with realistic job processing scenarios
+
+- **Configuration & Features**
+  - Feature flags: `postgres`, `mysql` for selective database support
+  - Workspace configuration for consistent dependency management
+  - Production-ready configuration with performance optimizations
+
+### Technical Details
+- Built on Tokio with async/await throughout the codebase
+- Uses database transactions for atomic job state changes
+- Type-safe job handling with `Result<()>` error propagation
+- SQLx for compile-time query checking and database abstraction
+- Generic over database types using `sqlx::Database` trait
+
+---
+
+## Release Links
+- [0.2.1](https://github.com/CodingAnarchy/hammerwork/releases/tag/v0.2.1)
+- [0.2.0](https://github.com/CodingAnarchy/hammerwork/releases/tag/v0.2.0)
+- [0.1.0](https://github.com/CodingAnarchy/hammerwork/releases/tag/v0.1.0)
+
+## Contributing
+Please see [CONTRIBUTING.md](CONTRIBUTING.md) for details on our code of conduct and the process for submitting pull requests.
+
+## License
+This project is licensed under the MIT License - see the [LICENSE-MIT](LICENSE-MIT) file for details.