feat(sgx): add Uptime Kuma to monitor mx services independently

Runs on sgx so alerts (via Gmail) still work even if mx is down.
Available at https://status.hoyer.world behind nginx with ACME cert.

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

systems/x86_64-linux/sgx/nginx.nix

@@ -32,5 +32,14 @@
         proxyWebsockets = true;
       };
     };
+    "status.hoyer.world" = {
+      enableACME = false;
+      useACMEHost = "internal.hoyer.world";
+      forceSSL = true;
+      locations."/" = {
+        proxyPass = "http://127.0.0.1:4000";
+        proxyWebsockets = true;
+      };
+    };
   };
 }