Add opt-in EMBED_KEY passwordless sign-in and iframe-friendly session cookie

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -12,6 +12,8 @@ services:
       OPENAUDIBLE_PASSWORD: ${OPENAUDIBLE_PASSWORD}
       PLAYER_USER: ${PLAYER_USER}                    # default account, created on first start
       PLAYER_PASSWORD: ${PLAYER_PASSWORD}
+      EMBED_KEY: ${EMBED_KEY:-}      # optional: enables /auto?key=... passwordless sign-in (serve over HTTPS)
+      EMBED_USER: ${EMBED_USER:-}    # which account it signs in as (default: first account)
     volumes:
       - player-data:/data    # accounts, sessions, listening progress