Milestone 2: ingestion daemon driving the stream

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>

ingest/Dockerfile

@@ -0,0 +1,12 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+# Milestone 2 uses the standard library only; third-party dependencies
+# (httpx, yt-dlp, feedparser…) will be added in later milestones.
+COPY radieo/ ./radieo/
+
+ENV PYTHONUNBUFFERED=1
+EXPOSE 8080
+
+CMD ["python", "-m", "radieo"]