Transcode recordings from WebM to MP3 via ffmpeg

[AevumDecessus]

Summary

Server-side transcode uploaded WebM/Opus recordings to MP3 for playback compatibility on older devices that do not support WebM (pre-2021 iOS, legacy browsers).

Requirements

• ffmpeg as system dependency in Docker image
• Transcode WebM/Opus to MP3 (128kbps mono)
• Extract duration via ffprobe if not already set
• Store MP3 alongside or replacing the original WebM
• Clean up source WebM after successful transcode (optional, could keep both)

Technical notes

• Can run synchronously on upload for simplicity, or as a background task if performance is a concern
• ffmpeg command: ffmpeg -i input.webm -ac 1 -ab 128k -f mp3 output.mp3
• Duration extraction: ffprobe -v quiet -show_entries format=duration -of csv=p=0 output.mp3
• Playback view would need to serve the MP3 instead of (or alongside) the WebM

Context

WebM/Opus has broad support in 2026 browsers (Chrome, Firefox, Edge, Safari 14.1+). This is a fallback for edge cases like older donated hospital devices. Not required for MVP launch.

Acceptance criteria

• ffmpeg added to Docker image
• Transcoded MP3 created after upload
• Duration extracted and saved to model
• Playback view serves compatible format