feat: Shared code volume for nginx and app containers

- Add app-code volume to both nginx and app containers
- App container clones code once into shared volume
- Nginx container uses shared volume (read-only) and waits for code
- Update entrypoint.sh to handle storage-preserving git clone
- Fix nginx configuration to use app:9000 for PHP-FPM

This eliminates duplicate code cloning and ensures both containers
always use the same code version from the shared volume.

deployment/stacks/application/docker-compose.yml

@@ -13,6 +13,7 @@ services:
       - APP_ENV=${APP_ENV:-production}
       - APP_DEBUG=${APP_DEBUG:-false}
       - APP_URL=${APP_URL:-https://michaelschiemer.de}
+      - APP_KEY=${APP_KEY:-}
       # Git Repository (optional - if set, container will clone/pull code on start)
       - GIT_REPOSITORY_URL=${GIT_REPOSITORY_URL:-}
       - GIT_BRANCH=${GIT_BRANCH:-main}
@@ -39,6 +40,7 @@ services:
       - QUEUE_DRIVER=redis
       - QUEUE_CONNECTION=default
     volumes:
+      - app-code:/var/www/html
       - app-storage:/var/www/html/storage
       - app-logs:/var/www/html/storage/logs
       - /etc/timezone:/etc/timezone:ro