fix(deployer): env_file statt leerem env-Passthrough für Webhook-Secret

${DEPLOY_WEBHOOK_SECRET} war leer (keine Root-.env), deployer-Container
hat secret nicht erhalten → HTTP 403. Nun direkt aus src/.env gelesen.

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

docker-compose.staging.yml

@@ -169,8 +169,7 @@ services:
     restart: unless-stopped
     ports:
       - "9000:9000"
-    environment:
-      DEPLOY_WEBHOOK_SECRET: ${DEPLOY_WEBHOOK_SECRET}
+    env_file: src/.env
     volumes:
       - ./docker/webhook/hooks.json:/etc/webhook/hooks.json:ro
       - ./docker/webhook/entrypoint.sh:/entrypoint.sh:ro