feat: optional YouTube Music source (cookies volume + wizard step)

Adds a step_youtube wizard prompt (enable + cookies host folder), the @YOUTUBE_VOLUME@ token in templates/compose/backend.yml substituted in compose_gen, a YOUTUBE_ENABLED/cookies env block, ensure_youtube_dir in lifecycle, en/ru strings, and a README step.

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

templates/compose/backend.yml

@@ -6,6 +6,7 @@
     volumes:
       - ${MEDIA_HOST_PATH}:${MEDIA_PATH}
       - transcode_cache:${TRANSCODE_CACHE_PATH}
+@YOUTUBE_VOLUME@
 @API_PORTS@
 @API_DEPENDS@
 
@@ -17,4 +18,5 @@
     volumes:
       - ${MEDIA_HOST_PATH}:${MEDIA_PATH}
       - transcode_cache:${TRANSCODE_CACHE_PATH}
+@YOUTUBE_VOLUME@
 @WORKER_DEPENDS@