refactor(docker): replace env_file with explicit environment variables

- Remove env_file references from docker-compose.yml and docker-compose.prod.yml
- Add explicit environment variable forwarding for all services
- Use TRAEFIK_NETWORK env var in docker-compose.traefik.yml instead of hardcoded name
- Add VITE_* frontend variables to .env.example and deploy/.env.example
- Add missing production variables (TRAEFIK_ENTRYPOINT, TRAEFIK_CERT_RESOLVER) to deploy/.env.example

This makes environment configuration more explicit and easier to manage
across different deployment scenarios (local dev, CI/CD, Portainer).

.env.example

@@ -27,6 +27,11 @@ TRAEFIK_ENTRYPOINT=websecure
 TRAEFIK_CERT_RESOLVER=letsencrypt
 TOOL_SUBDOMAIN_PATTERN={tool}-{project}-{user}.tools.localhost
 
+# Frontend build-time variables (passed to web container)
+VITE_API_URL=http://localhost:8000
+VITE_OIDC_ISSUER=https://auth.example.com/application/o/headquarter/
+VITE_OIDC_CLIENT_ID=your-client-id
+
 # Secrets (generate strong random values for production)
 SECRET_ENCRYPTION_KEY=change-me-in-production