radio: /srv/betoniarka paths, in-container Icecast, drop ffmpeg.pref

Per review of the first radio-container cut:

- radio_conjurer.liq: replace the cargo-culted /home/pi/Conjurer and
  /home/pi/MediaFolder paths with the container layout -
  /srv/betoniarka/data (playlists, script.params, persistence/radio logs),
  /srv/betoniarka/music (library + emergency track) and
  /srv/betoniarka/secrets/icecast_credentials.json;
  interactive.persistent now uses an absolute path;
  output.icecast targets host="localhost" (icecast lives in the container)
- Icecast2 now runs INSIDE the radio container: entrypoint renders
  /etc/icecast2/icecast.xml from docker/icecast.xml.tpl, filling
  source/admin/relay passwords from the secret provisioned at install time
  (same pattern as the bot's netrc); optional admin_password/relay_password
  fields default to password; icecast starts as its unprivileged user;
  port 8000 exposed for listeners; extra_hosts hack removed
- compose.radio.yaml: same-path mounts for /srv/betoniarka/{data,music,secrets}
- ffmpeg.pref removed from the repo (the pin only mattered against the
  RPi OS repo; bookworm ships the right FFmpeg 5.x natively) - verdict
  preserved in the docs and the legacy installer note updated
- docs: updated radio section (volumes, secret provisioning, wire-up)

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

docker/compose.radio.yaml

@@ -1,4 +1,4 @@
-# Radio (Liquidsoap) VM. Run from the repository root:
+# Radio (Liquidsoap + Icecast) VM. Run from the repository root:
 #   docker compose -f docker/compose.radio.yaml up -d --build
 #
 # Version pins are build args - override via environment, e.g.:
@@ -21,19 +21,21 @@ services:
       # host     = mount the host pulse socket below and set PULSE_SERVER
       PULSE_MODE: ${PULSE_MODE:-internal}
       # PULSE_SERVER: unix:/tmp/pulseaudio.socket
-    extra_hosts:
-      # radio_conjurer.liq streams to host="radio" - point it at icecast.
-      - "radio:${ICECAST_HOST_IP:-192.168.1.12}"
+      # Hostname icecast reports in its status/YP pages:
+      ICECAST_HOSTNAME: ${ICECAST_HOSTNAME:-localhost}
     ports:
+      - "8000:8000"     # icecast - listeners tune in here
       - "54321:54321"   # harbor /skip - the bot's CONJURER_RADIO_HARBOR
       - "9999:9999"     # interactive harbor (keep LAN-only!)
     volumes:
-      # Playlists, logs, script.params, icecast_credentials.json, the script
-      # itself (seeded on first run) - paths mirror the ones hardcoded in
-      # radio_conjurer.liq so it runs unmodified.
-      - /srv/radio/data:/home/pi/Conjurer
+      # Playlists, logs, script.params and the script itself (seeded on first
+      # run) - same paths inside and outside the container.
+      - /srv/betoniarka/data:/srv/betoniarka/data
       # The mp3 library. Writable because the entrypoint seeds a silent
       # emergency-fallback mp3 when the hardcoded single() file is missing.
-      - /srv/radio/music:/home/pi/MediaFolder/mp3
+      - /srv/betoniarka/music:/srv/betoniarka/music
+      # icecast_credentials.json - provision at install like the other
+      # services' secrets (a CHANGE_ME placeholder is seeded if absent).
+      - /srv/betoniarka/secrets:/srv/betoniarka/secrets
       # PULSE_MODE=host: uncomment and adjust
       # - /tmp/pulseaudio.socket:/tmp/pulseaudio.socket