Add maintenance page support during deployment

[dannywillems]

Motivation

During deployment, catapulta runs docker compose down which stops all
containers including Caddy. Users visiting the site during this window
see a browser-level ERR_CONNECTION_REFUSED error, which is a poor
experience.

Problems

1. Caddy is part of the docker-compose stack, so it goes down with app
   containers.
2. There is no mechanism to show a user-friendly page during deployment
   downtime.
3. The depends_on: service_healthy on Caddy prevents it from running
   independently of app containers.

Suggested solution

Add a maintenance_page(html_path) method to Caddy that:

1. Reads a user-provided HTML file and embeds it into the Caddyfile as
   a handle_errors block responding to 502/503/504 errors.
2. Removes depends_on from the Caddy service in docker-compose so
   Caddy can stay running while app containers restart.
3. During cmd_deploy, only stops app containers (docker compose rm -sf <apps>) instead of docker compose down, keeping Caddy alive
   to serve the maintenance page.
4. Before stopping apps, updates the Caddyfile on the remote host and
   reloads Caddy config so the maintenance page is active.

The HTML content is fully user-controlled, not hardcoded in catapulta.

Usage:

let caddy = Caddy::new()
    .route("/api/*", api.upstream())
    .route("", web.upstream())
    .maintenance_page("deploy/maintenance.html")
    .gzip()
    .security_headers();