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>

docker-compose.yml

@@ -1,9 +1,21 @@
 services:
+  ingest:
+    build: ./ingest
+    image: radieo-ingest
+    volumes:
+      - ./cache:/cache          # volume partagé avec le stream
+    environment:
+      - RADIEO_CACHE_DIR=/cache
+      - RADIEO_HTTP_PORT=8080
+    restart: unless-stopped
+
   stream:
     build: ./stream
     image: radieo-stream
+    depends_on:
+      - ingest
     ports:
-      - "8000:8000"          # flux HTTP : http://localhost:8000/radio.mp3
+      - "8000:8000"             # flux HTTP : http://localhost:8000/radio.mp3
     volumes:
-      - ./cache:/cache:ro    # jalon 1 : lecture seule, rempli à la main
+      - ./cache:/cache:ro       # lecture seule : secours + résolution des chemins
     restart: unless-stopped