refactor(beszel): deduplicate alerts query input and document systemId option

- Memoize alerts query input to avoid triple repetition
- Add system selector to docs configuration items

apps/docs/docs/widgets/beszel-system-stats/index.ts

@@ -8,6 +8,12 @@ export const beszelSystemStatsWidget: WidgetDefinition = {
   path: "../../widgets/beszel-system-stats",
   configuration: {
     items: [
+      {
+        name: "System",
+        description: "The Beszel system to display stats for. Searchable select populated from the integration.",
+        values: { type: "text" },
+        defaultValue: "First available system",
+      },
       {
         name: "Time period",
         description: