[sivert-io]

The worker half of Gryt-chat/server#277.

• A quarantined video under uploads/ is a chat video. runChatVideoJob writes it out again as uploads/{id}.mp4 with a poster, or refuses it.
• Docker (transcodeChatVideo): AV1 (SVT, preset 10, CRF 38) and stereo AAC 128k in MP4. Every frame is scaled into 1280px keeping its shape, at most 30 fps (-fpsmax). Only the first audio track is kept, and only if there is one. No metadata, chapters, subtitles or data streams. Ten-minute limit.
• Desktop: chatVideoOnDesktop asks the app's sandbox for a chatvideo job (client PR alongside), and checkChatVideoAnswer checks the answer like every other one.
• The worker claims chatvideo-v1 where the jail or the app sandbox works.
• Dockerfile: ffmpeg gains audio. Decoders aac, mp3, opus, vorbis, flac and PCM; the matching parsers; the AAC encoder; swresample (was disabled); and the aresample, aformat and anull filters. Still the same jail, protocols and demuxers.

Tested in the built image: withaudio.mp4, H.264, the resizing WebM, HEVC, MOV, a portrait phone video (comes out 720×1280) and an 8K VP9 all converted in under a second, with AAC audio where the source had sound. HLS-as-mp4, random bytes and a truncated MP4 were refused. 61 unit tests pass.

What to look at: the bigger ffmpeg surface (audio decoders and swresample) in the Dockerfile. It runs in the same jail.

Review-required (image-worker).

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