Add admin handler for health checks and preview regeneration

Commit notes

This commit introduces an AdminHandler that provides endpoints for health checks and regenerating audio previews and sequencer files. The health check now includes ffmpeg availability verification. Additionally, the health check endpoint in the main application has been updated to use the new AdminHandler. An Aptfile is added to manage ffmpeg installation, and the content type for audio responses is updated to 'audio/opus' for better compatibility.