Add guschin.info VirtualHost alongside xmpp.guschin.info

Users can now register as name@guschin.info or name@xmpp.guschin.info.
Added Let's Encrypt cert mounting and install for guschin.info domain.
Refactored entrypoint cert install into reusable function.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -25,6 +25,8 @@ services:
       - ./data/prosody/configuration:/etc/prosody/conf.d
       - /etc/letsencrypt/live/xmpp.guschin.info:/etc/prosody/certs/letsencrypt/live/xmpp.guschin.info:ro
       - /etc/letsencrypt/archive/xmpp.guschin.info:/etc/prosody/certs/letsencrypt/archive/xmpp.guschin.info:ro
+      - /etc/letsencrypt/live/guschin.info:/etc/prosody/certs/letsencrypt/live/guschin.info:ro
+      - /etc/letsencrypt/archive/guschin.info:/etc/prosody/certs/letsencrypt/archive/guschin.info:ro
     restart: unless-stopped
     mem_limit: 200M
     healthcheck: