feat(deploy): support a reverse proxy already on the server

[xAlcahest]

Summary

deploy.sh only knew one way to face the internet, our own Caddy on 80 and 443. A server that already runs Nginx Proxy Manager or Traefik owns those ports and reaches its services over a shared Docker network, so docker-compose.proxy.yml joins the app to that network and deploy.sh switches to it when PROBUS_PROXY_NETWORK is set, skipping the Caddy-only checks and verifying the network exists on the host first.

Changes

• docker-compose.proxy.yml: overlay attaching the app to an external proxy network.
• scripts/deploy.sh: mode switch on PROBUS_PROXY_NETWORK, network check, overlay shipped alongside the compose file.
• .env.example: PROBUS_PROXY_NETWORK documented next to the TLS settings.
• DEPLOY.md: the two modes in step 4, plus the smoke check count and file list brought up to date.