feat(blog): add retention metadata to articles collection

Monthly purge schedule with 90-day post-deletion hard-delete window.
Part of compliance backfill (Story 05).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/blog/src/integrations/cms/collections/articles.ts

@@ -3,6 +3,16 @@ import { slugifyIfMissing } from "@repo/core-shared/payload";
 
 export const articles: CollectionConfig = {
   slug: "articles",
+  custom: {
+    retention: {
+      purgeSchedule: "monthly",
+      postDeletion: {
+        duration: "P90D",
+        trigger: "after-deletion",
+        action: "hard-delete",
+      },
+    },
+  },
   admin: {
     useAsTitle: "title",
     defaultColumns: ["title", "status", "author", "updatedAt"],