Cobalt 11.7.1 origin muxer repair

Upstream: https://github.com/imputnet/cobalt
Upstream revision: a636575b09de1fc55d9b8cd98cac88f5f2f16b42
Upstream source file: api/src/stream/ffmpeg.js
Upstream license: GNU Affero General Public License version 3 or later (LICENSE).
Copyright remains with imput and the upstream contributors.

Modified 2026-11-08 by Bento maintainers under the same license:
- A single-input video remux accepts an absent audio stream; video remains required.
- Known FFmpeg output containers receive the corresponding HTTP Content-Type.
- Audio conversion, authentication, URLs, budgets and runtime limits are unchanged.

This directory contains public upstream material and a generic vendor repair only.
It contains no private Bento application code, deployment configuration or keys.
The corresponding-source bundle must remain publicly available, with a clear
source offer to remote users, before this modified server is deployed publicly.
See README.md for the complete-source packaging and standalone build procedure.
