fix: CI deploy parity with compose — media env, volume, healthcheck

A missing env var aborts supervisord's %(ENV_x)s config expansion and
takes every program down, which is why the media-player merge failed
CI (run 151): rebuild.yml's docker run did not pass MEDIA_DIR /
MEDIA_SERVER_PORT / YTDLP_COOKIES.

- supervisord.conf: media-server inherits those vars from the container
  environment instead of hard-requiring them (media_server.py has safe
  defaults for all three).
- rebuild.yml: mount the media library volume, pass the media env, and
  extend the healthcheck to the media server healthz.
- compose: YTDLP_COOKIES defaults to the conventional /data/media/
  cookies.txt path (ignored unless the file exists).

Parity verified: container started with CI's exact env (no media vars)
— all five programs RUNNING.

docker-compose.yml

@@ -20,7 +20,7 @@ services:
       VOICE_DEFAULT_USER: "${VOICE_DEFAULT_USER:-shane}"
       MEDIA_DIR: "/data/media"
       MEDIA_SERVER_PORT: "${MEDIA_SERVER_PORT:-8092}"
-      YTDLP_COOKIES: "${YTDLP_COOKIES:-}"
+      YTDLP_COOKIES: "${YTDLP_COOKIES:-/data/media/cookies.txt}"
     volumes:
       - ./livekit.yaml:/etc/livekit.yaml:ro
       - ./certs:/etc/voice/certs