Cobalt 11.7.1 muxer5 and audio normalization

Upstream: https://github.com/imputnet/cobalt
Upstream revision: a636575b09de1fc55d9b8cd98cac88f5f2f16b42
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:
- api/src/stream/ffmpeg.js retains silent-video remuxing, explicit known output MIME
  types, estimate-before-spawn/output-pipe sequencing, cancellation, and advisory
  failed-size-estimate fixes. It now shares a single FFmpeg process-capacity gate
  with audio normalization.
- api/src/stream/internal-hls.js identifies parsed rewritten HLS playlists as
  application/vnd.apple.mpegurl without relaxing protocol or extension checks.
- api/src/core/api.js adds an opt-in local audio-normalization route behind the
  existing API key authentication and request limiter.
- New api/src/core/normalize-audio.js implements bounded upload-to-audio processing,
  cancellation, process termination and cleanup, without fetching user URLs.
  FFmpeg cache input supports seeking with bounded read-ahead and an immediately
  unlinked file in a capped tmpfs; no persistent upload is retained.
- New api/src/stream/process-capacity.js provides shared single-process admission.

This directory contains public upstream source and generic modifications only.
It contains no private application source, deployment configuration, or credentials.
The exact complete corresponding source must be available through a prominent
working source offer to remote users. Retain earlier deployed source versions and
checksums when publishing an updated source index. See README.md for rebuilds.