feat: add Loki datasource configuration for Grafana

hosts/web-arm/modules/grafana/datasources/loki.nix

@@ -0,0 +1,17 @@
+{ lib, pkgs, config, ... }:
+{
+  services.grafana.provision.datasources.settings.datasources = [
+    {
+      name = "Loki";
+      uid = "loki-datasource-uid"; # Stable UID
+      type = "loki";
+      url = "http://localhost:3100"; # Loki's local address
+      access = "proxy"; # Grafana proxies requests
+      isDefault = false; # Can be set to true if desired
+      jsonData = {
+        # Loki-specific options (e.g., derivedFields) can go here
+      };
+      editable = false; # Recommended for provisioned datasources
+    }
+  ];
+}