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/lifecycle.sh

@@ -36,6 +36,17 @@ ensure_media_dir() {
     mkdir -p "$p"
 }
 
+ensure_youtube_dir() {
+    # YOUTUBE_COOKIES_HOST_PATH is only present when the YouTube source is
+    # enabled; create the folder so the bind mount has a real host directory
+    # (the user drops cookies.txt into it later).
+    local p
+    p="$(grep -E '^YOUTUBE_COOKIES_HOST_PATH=' "$ENV_FILE" | cut -d= -f2-)"
+    [[ -n "$p" ]] || return 0
+    [[ "$p" = /* ]] || p="${BOOTSTRAP_DIR}/${p#./}"
+    mkdir -p "$p"
+}
+
 lifecycle_pull() {
     ui_info "$(t pull_images "$(grep -E '^MCMA_IMAGE_TAG=' "$ENV_FILE" | cut -d= -f2-)")"
     ui_dim "$(t pull_hint)"
@@ -64,6 +75,7 @@ sys.exit(1)
 lifecycle_start() {
     local create_admin="${1:-no}"
     ensure_media_dir
+    ensure_youtube_dir
     lifecycle_pull
 
     local deps; deps="$(backing_services)"
@@ -111,7 +123,7 @@ lifecycle_update() {
     ui_ok "$(t done_title)"
 }
 
-lifecycle_up()     { ensure_media_dir; dc up -d; }
+lifecycle_up()     { ensure_media_dir; ensure_youtube_dir; dc up -d; }
 lifecycle_down()   { dc down; }
 lifecycle_logs()   { dc logs -f --tail=100; }