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.

.gitea/workflows/rebuild.yml

@@ -26,7 +26,7 @@ jobs:
             --name voice \
             --network host \
             --restart unless-stopped \
-            --health-cmd='curl -sk https://localhost:${WEB_PORT:-9178}/ -o /dev/null && curl -s http://localhost:7880/ -o /dev/null' \
+            --health-cmd='curl -sk https://localhost:${WEB_PORT:-9178}/ -o /dev/null && curl -s http://localhost:7880/ -o /dev/null && curl -s http://localhost:${MEDIA_SERVER_PORT:-8092}/media/api/healthz -o /dev/null' \
             --health-interval=15s \
             --health-timeout=5s \
             --health-retries=3 \
@@ -34,6 +34,7 @@ jobs:
             -v /mnt/user/dev/hope-voice-api/certs:/etc/voice/certs \
             -v /mnt/user/dev/hope-voice-api/skills:/skills \
             -v /mnt/user/dev/hope-voice-api/livekit.yaml:/etc/livekit.yaml:ro \
+            -v /mnt/user/dev/hope-voice-api/media:/data/media \
             -e LIVEKIT_URL=http://localhost:7880 \
             -e LIVEKIT_API_KEY=devkey \
             -e LIVEKIT_API_SECRET=hopevoice-devsecret-2026-lan-only \
@@ -48,6 +49,9 @@ jobs:
             -e WEB_PORT=9178 \
             -e VOICE_USERS_FILE=/etc/hope/users.json \
             -e VOICE_DEFAULT_USER=shane \
+            -e MEDIA_DIR=/data/media \
+            -e MEDIA_SERVER_PORT=8092 \
+            -e YTDLP_COOKIES=/data/media/cookies.txt \
             hope-voice-api-voice:latest
 
       - name: Verify deployment