feat(mx): add ntfy-sh push notification service

Self-hosted at ntfy.hoyer.xyz with deny-all default access.
After deploying, create a user with: ntfy user add --role=admin harald

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

systems/x86_64-linux/mx/default.nix

@@ -14,6 +14,7 @@
     ./nextcloud.nix
     ./nextcloud-claude-bot
     ./nginx.nix
+    ./ntfy.nix
     ./postgresql.nix
     ./rspamd.nix
     ./rustdesk.nix