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/compose_gen.sh

@@ -31,6 +31,13 @@ generate_compose() {
         webui_ports="    ports:${nl}      - \"\${WEBUI_PORT}:80\""
     fi
 
+    # YouTube cookies volume: mounted into api+worker only when the source is
+    # enabled. The host path comes from .env.deploy (YOUTUBE_COOKIES_HOST_PATH).
+    local youtube_volume=""
+    if [[ "$CFG_YOUTUBE" == "yes" ]]; then
+        youtube_volume="      - \${YOUTUBE_COOKIES_HOST_PATH}:/data/youtube"
+    fi
+
     # Caddy publish ports: 80/443 with a domain (auto-HTTPS), else plain HTTP.
     if [[ "$CFG_HTTPS" == "yes" ]]; then
         caddy_ports="      - \"80:80\"${nl}      - \"443:443\""
@@ -41,6 +48,7 @@ generate_compose() {
     # -- assemble ---------------------------------------------------------
     local backend webui caddy
     backend="$(_frag backend.yml)"
+    backend="${backend//@YOUTUBE_VOLUME@/$youtube_volume}"
     backend="${backend//@API_PORTS@/$api_ports}"
     backend="${backend//@API_DEPENDS@/$api_depends}"
     backend="${backend//@WORKER_DEPENDS@/$worker_depends}"