docker: Liquidsoap radio container (opam-built, version-selectable)

Dockerises the radio_conjurer.liq environment on Debian bookworm:

- Dockerfile.radio builds Liquidsoap through opam with build args for
  OPAM_VERSION (static binary), OCAML_VERSION (default 4.14.2; 2.1.x needs
  OCaml 4.x - prod ran 4.13.0, pass it for parity) and
  LIQUIDSOAP_VERSION (default 2.1.4, matching the script)
- LIQ_OPAM_PACKAGES installs exactly the features the script uses:
  mad+lame (mp3), cry (icecast), taglib (tags/replaygain), pulseaudio
  (mic/out), samplerate, inotify (watch-reload), ffmpeg
- ffmpeg.pref verdict: the Pi pin forced Debian's FFmpeg 5.x family
  (libavcodec59...) over RPi OS repo builds because the ocaml-ffmpeg
  bindings are compiled against those sonames; bookworm ships them
  natively so the container needs no pin (documented, not copied)
- pulse-access/audio group memberships baked in (were manual on the Pi);
  PULSE_MODE=internal runs a system-wide pulse with a null sink so the
  unmodified script works on a headless VM (mic = silence), host mode
  mounts the real socket, none for edited scripts
- mounts mirror the script's hardcoded paths (/home/pi/Conjurer,
  /home/pi/MediaFolder/mp3) so radio_conjurer.liq runs unmodified;
  entrypoint seeds script+params on first run (never overwrites), creates
  missing playlists, seeds placeholder icecast credentials (loud warning)
  and generates a silent emergency mp3 when the single() file is missing
- compose.radio.yaml: extra_hosts maps the script's host="radio" to
  ICECAST_HOST_IP; exposes 54321 (bot's RADIO_HARBOR /skip) and 9999
- docs: radio section with build args, pulse modes and the ffmpeg.pref /
  groups verdicts

Part 2 of this PR (splitting bot<->radio comms out of the musician into a
radio+betoniarka container) follows on this branch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/entrypoint.radio.sh

@@ -0,0 +1,66 @@
+#!/bin/sh
+# Prepare the radio volumes so radio_conjurer.liq boots on a fresh VM, then
+# start (or point at) PulseAudio and exec liquidsoap. Existing files are never
+# overwritten - live-edited script/params/playlists always win.
+set -e
+
+DATA="${RADIO_DATA_DIR:-/home/pi/Conjurer}"
+MUSIC="${RADIO_MUSIC_DIR:-/home/pi/MediaFolder/mp3}"
+mkdir -p "$DATA" "$MUSIC"
+
+# Seed the script + persistent interactive params from the image on first run.
+[ -e "$DATA/radio_conjurer.liq" ] || cp /app/radio_conjurer.liq "$DATA/"
+if [ ! -e "$DATA/script.params" ]; then
+    if [ -e /app/script.params ]; then cp /app/script.params "$DATA/"; else : > "$DATA/script.params"; fi
+fi
+
+# Playlists + logs the script watches/writes; empty files keep it happy until
+# the musician/betoniarka populate them.
+for f in all_playlist.playlist priority_queue.playlist hit.playlist \
+         request.playlist jingles.playlist persistence.log; do
+    [ -e "$DATA/$f" ] || : > "$DATA/$f"
+done
+
+# Icecast credentials are a secret - never baked into the image. Seed a
+# placeholder so the script can parse the JSON, but shout about it.
+if [ ! -e "$DATA/icecast_credentials.json" ]; then
+    printf '{\n"password" : "CHANGE_ME"\n}\n' > "$DATA/icecast_credentials.json"
+    echo "WARNING: seeded placeholder icecast_credentials.json - the icecast" >&2
+    echo "         output will be rejected until you set the real password!" >&2
+fi
+
+# single() aborts the whole script when its file is missing; guarantee the
+# emergency fallback exists (5s of silence beats a dead radio).
+EMERGENCY="$MUSIC/Youtube/Dr. Peacock - Trip to Ireland [GvrvQTUbUcA].mp3"
+if [ ! -e "$EMERGENCY" ]; then
+    mkdir -p "$MUSIC/Youtube"
+    if ffmpeg -loglevel error -f lavfi -i anullsrc=r=44100:cl=stereo -t 5 \
+              -codec:a libmp3lame -q:a 9 "$EMERGENCY"; then
+        echo "WARNING: emergency track was missing - generated silent placeholder" >&2
+    else
+        echo "WARNING: could not create emergency track; single() may abort" >&2
+    fi
+fi
+
+# PulseAudio wiring:
+#   internal (default) - system-wide pulse inside the container with a null
+#                        sink (see /etc/pulse/system.pa); no sound hardware
+#                        needed, mic path reads silence.
+#   host               - use a socket mounted from the host; set PULSE_SERVER
+#                        (e.g. unix:/tmp/pulseaudio.socket) in the env file.
+#   none               - you edited the script to drop pulse in/out.
+case "${PULSE_MODE:-internal}" in
+    internal)
+        pulseaudio --system --daemonize=yes --disallow-exit --exit-idle-time=-1 \
+            || echo "WARNING: internal pulseaudio failed to start" >&2
+        export PULSE_SERVER="${PULSE_SERVER:-unix:/var/run/pulse/native}"
+        ;;
+    host)
+        [ -n "$PULSE_SERVER" ] || echo "WARNING: PULSE_MODE=host but PULSE_SERVER is unset" >&2
+        ;;
+    none)
+        ;;
+esac
+
+cd "$DATA"
+exec "$@"