feat: add subscription management with cancel and renew functionality

[Yunusabdul38]

closes: #22
closes: #46

Overview

This PR introduces a comprehensive subscription management system that enables full lifecycle control of user subscriptions. All core functionality has been implemented to automate subscription processes and improve user experience.

✅ Implemented Features

Core Functions

• Subscription Creation: Complete implementation of subscription creation with validation and error handling
• Renewal Management: Automated and manual renewal capabilities with proper state transitions
• Cancellation System: Clean subscription termination with appropriate cleanup processes
• Status Tracking: Real-time subscription status monitoring with accurate state management

Event System

• Lifecycle Events: Comprehensive event emission system covering all subscription state changes
  • Subscription created events
  • Renewal processed events
  • Cancellation completed events

🔧 Technical Implementation

Error Handling

• Robust validation for all subscription operations
• Comprehensive edge case handling for invalid data scenarios
• Graceful failure modes with appropriate error messaging

Code Quality

• Consistent code structure following established patterns
• Clear function naming and documentation
• Maintainable architecture for future enhancements

All acceptance criteria have been fully met:

• ✅ Subscriptions create successfully with proper validation
• ✅ Renewal function operates reliably with state management
• ✅ Cancellation properly terminates subscriptions and cleans up resources
• ✅Status tracking provides accurate real-time subscription information
• ✅ Events are emitted at every stage of the subscription lifecycle

🧪 Testing

• Unit tests covering all subscription functions
• Integration tests for end-to-end subscription workflows
• Edge case testing for error scenarios
• Event emission verification

📈 Impact

This implementation provides:

• Complete automation of subscription lifecycle management
• Enhanced user experience through reliable subscription handling
• Full transparency in subscription state changes through comprehensive events
• Solid foundation for future subscription feature enhancements

Ready for review and deployment! 🎉