deploy: PORT/HOST configurabili in .env + .env.example versionato

- .env: aggiunte vars PORT=8080, HOST=127.0.0.1, REGISTRY, TAG
- docker-compose.yml: usa ${PORT:-8080} sia per container env che per
  traefik loadbalancer.server.port (coerenza)
- .env.example: template versionato con valori default sicuri
  (.env resta in .gitignore, non committato)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -14,7 +14,7 @@ services:
     environment:
       IMAGES_DIR: /data/images
       HOST: 0.0.0.0
-      PORT: 8080
+      PORT: ${PORT:-8080}
     volumes:
       # Persistenza immagini tra restart (upload/selezione)
       - ./images:/data/images
@@ -28,7 +28,7 @@ services:
       - "traefik.http.routers.pm2d.entrypoints=websecure"
       - "traefik.http.routers.pm2d.tls=true"
       - "traefik.http.routers.pm2d.tls.certresolver=letsencrypt"
-      - "traefik.http.services.pm2d.loadbalancer.server.port=8080"
+      - "traefik.http.services.pm2d.loadbalancer.server.port=${PORT:-8080}"
 
       # Middleware: upload fino a 50MB (default Traefik bufferizza a 4MB)
       - "traefik.http.middlewares.pm2d-bodysize.buffering.maxRequestBodyBytes=52428800"