Add comprehensive Docker-based integration testing infrastructure

This commit implements end-to-end testing infrastructure for the Hammerwork
job queue library with PostgreSQL and MySQL database backends, including
comprehensive error fixes and code quality improvements.

## Major additions:

### Docker Infrastructure
- docker-compose.yml with PostgreSQL 16 and MySQL 8.0 services
- Health checks and persistent data volumes
- Database initialization scripts for both backends

### Integration Testing Framework
- Workspace configuration supporting multiple integration apps
- Shared test scenarios for both PostgreSQL and MySQL
- Database-specific tests for JSONB/JSON operations
- Comprehensive job lifecycle, retry, and worker pool testing
- Performance benchmarking and concurrent processing tests

### CI/CD Pipeline
- GitHub Actions workflow with matrix testing across databases
- Automated unit tests, integration tests, and performance benchmarks
- Security auditing and dependency vulnerability scanning

### Development Tooling
- Makefile with convenient development commands
- Shell scripts for local testing automation with colored output
- Database setup and cleanup utilities
- Support for selective testing (unit-only, database-specific)

## Code quality improvements:

### Compiler Error Fixes
- Fixed unused import warnings in integration tests
- Added missing JobStatus imports in feature-gated queue modules
- Updated examples to use correct SQLx Pool types
- Fixed Result type conflicts in async function signatures
- Proper type annotations for job handlers

### Library Enhancements
- Enhanced error handling with better error messages
- Improved tracing and logging integration
- Updated examples with proper database connection patterns
- Comprehensive test coverage for all core functionality

### Documentation
- Updated README with integration testing instructions
- Added development workflow documentation
- Comprehensive setup guides for local and CI environments

This infrastructure enables reliable testing across database backends and
provides a solid foundation for continuous integration and deployment.

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

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

Author: CodingAnarchy

.claude/settings.local.json

@@ -10,7 +10,9 @@
       "Bash(cargo check:*)",
       "Bash(cargo test:*)",
       "Bash(cargo:*)",
-      "Bash(find:*)"
+      "Bash(find:*)",
+      "Bash(chmod:*)",
+      "Bash(./scripts/test-local.sh:*)"
     ],
     "deny": []
   }