Add recording

[leszko]

Env variables

RECORDING_ENABLED

• Default: true
• Description: Enable/disable recording. When false, recording is disabled.

RECORDING_MAX_LENGTH

• Default: 1h
• Description: Maximum total recording length (sum of all segments). Recording stops when reached.
• Format: Supports 1h, 30m, 120s, or plain seconds (e.g., 3600)

RECORDING_STARTUP_CLEANUP_ENABLED

• Default: true
• Description: Enable/disable cleanup of recording files

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Assumptions / Comments

• The recording stream is sourced from the same place we send frames to the frontend.
  • This means that if a frame is dropped, it will also be missing from the recording (sorry, Ryan).
  • The main benefit is that we can piggy-back on our existing PTS calculation mechanism, and aiortc.contrib.media.MediaRecorder.
• Temporary recordings are stored in the OS temp directory.
• Both stream pause/resume and exporting a recording cause the backend to segment/clip the stream. The final recording is a concatenation of these segments when downloaded. This is mainly due to how pause handling works and the behavior of MediaRecorder, which needs to finalize the stream before exporting.
• The pause/resume feature adds significant complexity to recording logic. On the backend we keep sending the same frame while paused, but we don't want repeated identical frames in the final recording.
• Recording input may be tricky because of the pause/resume (we don't pause input stream!)
• We use ffmpeg (as Python library) for mp4 segments concatenation; I experimented with using AV library, but it happened to be surprisingly difficult (a lot of corner cases, etc.), so I ended up with ffmpeg. AV is still an option, but required more work and maintenance. One other option is to use the moviepy library

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fix #56