Add the Python `ingest` container exposing `GET /next`, which returns the
next track as an annotated Liquidsoap URI (or an empty body when nothing is
ready). Liquidsoap switches from a static playlist to a `request.dynamic`
source pulling from the daemon, with the local cache as fallback and mksafe
for guaranteed continuous output.
For now the daemon just cycles through the files already in the cache; the
download providers (Navidrome, yt-dlp, ListenBrainz) come in later milestones.
Also commit the implementation plan (PLAN.md).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
