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>

data/prosody/configuration/prosody.cfg.lua

@@ -1,7 +1,7 @@
 -- Prosody XMPP Server Configuration
 -- Domain: xmpp.guschin.info
 
-admins = { "admin@xmpp.guschin.info" }
+admins = { "admin@guschin.info", "admin@xmpp.guschin.info" }
 
 modules_enabled = {
 	-- Generally required
@@ -69,7 +69,14 @@ archive_expires_after = "never"
 -- Certificates directory (global, before VirtualHost)
 certificates = "/etc/prosody/certs"
 
--- Virtual host definition
+-- Primary virtual host (user@guschin.info)
+VirtualHost "guschin.info"
+    ssl = {
+        key = "/etc/prosody/certs/guschin.info.key";
+        certificate = "/etc/prosody/certs/guschin.info.crt";
+    }
+
+-- Secondary virtual host (user@xmpp.guschin.info)
 VirtualHost "xmpp.guschin.info"
     ssl = {
         key = "/etc/prosody/certs/xmpp.guschin.info.key";
@@ -77,6 +84,6 @@ VirtualHost "xmpp.guschin.info"
     }
 
 -- Component for MUC (Multi-User Chat)
-Component "muc.xmpp.guschin.info" "muc"
+Component "muc.guschin.info" "muc"
     modules_enabled = { "muc_mam" }
     storage = "sql"