lamassu: register module in modules.nix

Without this, the services.lamassu-server option doesn't exist.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/modules.nix

@@ -28,6 +28,7 @@
     ./joinmarket.nix
     ./joinmarket-ob-watcher.nix
     ./hardware-wallets.nix
+    ./lamassu-lnbits.nix
 
     # Support features
     ./versioning.nix