fix: Stockfish via npm-Paket statt Alpine apk

- stockfish als npm dependency (kein apk add nötig, portabel)
- Dockerfile: nur noch git via apk, kein stockfish
- docker-compose.yml: version-Zeile entfernt (veraltet)
- Controller auf npm-Package-API umgeschrieben (postMessage/onmessage)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -1,5 +1,3 @@
-version: "3.8"
-
 services:
   michess:
     build: .
@@ -20,8 +18,6 @@ services:
       CORS_ORIGIN: ${CORS_ORIGIN:-http://localhost:3000}
       # Admin setup: user with this email gets admin role on startup
       ADMIN_EMAIL: ${ADMIN_EMAIL:-}
-      # Stockfish binary path (stockfish is installed in the image)
-      STOCKFISH_PATH: stockfish
       # Git update path (used by admin panel update button)
       APP_DIR: /opt/michess
     ports: