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>

lib/env_gen.sh

@@ -93,5 +93,24 @@ generate_env() {
         } >>"$ENV_FILE"
     fi
 
+    # -- YouTube Music source ---------------------------------------------
+    # COOKIES_HOST_PATH is consumed by the compose volume; COOKIES_PATH is the
+    # in-container path the backend reads (only used when the file is present).
+    if [[ "${CFG_YOUTUBE:-no}" == "yes" ]]; then
+        {
+            echo ""
+            echo "# -- YouTube Music source -----------------------------------------------"
+            echo "YOUTUBE_ENABLED=true"
+            echo "YOUTUBE_COOKIES_HOST_PATH=${CFG_YOUTUBE_COOKIES_HOST_PATH}"
+            echo "YOUTUBE_COOKIES_PATH=/data/youtube/cookies.txt"
+        } >>"$ENV_FILE"
+    else
+        {
+            echo ""
+            echo "# -- YouTube Music source (disabled) ------------------------------------"
+            echo "YOUTUBE_ENABLED=false"
+        } >>"$ENV_FILE"
+    fi
+
     chmod 600 "$ENV_FILE"
 }