feat: Complete WaveSurfer integration with enhanced audio playback and backend improvements

Commit notes

- Enhanced the WaveformPlayer component to support dark mode, interactive seeking, time display, and auto-rewind features. - Updated backend API responses to include preview URLs for samples, improving the user experience in the frontend. - Introduced a new cache invalidation endpoint to ensure fresh data is served after projection updates. - Improved drum classification logic and UMAP projection for better audio feature analysis.

These changes significantly enhance audio playback capabilities and streamline sample processing in the application.