Implement complete Beatstep Marketplace MVP with backend and frontend enhancements

Commit notes

- Added comprehensive implementation summary and features documentation for the Beatstep Marketplace MVP. - Introduced new backend models for Purchase, Pattern, SequencerProject, DMCANotice, DMCACounterNotice, ContentFlag, and AnalyticsEvent. - Enhanced existing models with marketplace-specific fields such as price, slug, and tags. - Implemented new handlers for billing, checkout, market, moderation, and analytics, including detailed test coverage. - Updated frontend routes and components to support marketplace functionalities, including pack purchasing and library access. - Integrated analytics tracking for user interactions and purchase events. - Updated pnpm lockfile and package dependencies for improved project management. - Added utility functions for slug generation and audio preview handling. - Ensured production readiness with comprehensive error handling and validation across the application.