fix(dev-deploy): probe Mailpit /mailpit/livez under MP_WEBROOT

MP_WEBROOT=/mailpit prefixes every Mailpit HTTP route, including the
/livez health endpoint. The container healthcheck still probed
http://localhost:8025/livez, which now 404s, so Mailpit reported
unhealthy; the backend depends_on it with condition: service_healthy
and never started, cascading to the gateway and Caddy and failing
`docker compose up --wait`. Point the healthcheck at /mailpit/livez.

tools/dev-deploy/docker-compose.yml

@@ -84,7 +84,7 @@ services:
     volumes:
       - galaxy-dev-mailpit-config:/etc/mailpit:ro
     healthcheck:
-      test: ["CMD", "wget", "-q", "-O-", "http://localhost:8025/livez"]
+      test: ["CMD", "wget", "-q", "-O-", "http://localhost:8025/mailpit/livez"]
       interval: 3s
       timeout: 3s
       retries: 30