Replace Uptime Kuma with Gatus for declarative uptime monitoring

Gatus is configured entirely via YAML (mapped from Nix attrsets),
making nix-config the single source of truth for all monitoring
config instead of Uptime Kuma's web UI/SQLite database.

common/server/default.nix

@@ -17,6 +17,6 @@
     ./actualbudget.nix
     ./unifi.nix
     ./ntfy.nix
-    ./uptime-kuma.nix
+    ./gatus.nix
   ];
 }