fix(blog): hide drafts at the access layer, and back the --drop claim

Review findings on #140.

Drafts were reachable. Posts granted unconditional public read and the
_status filter lived only in the pages that query the collection — which
is a convenience, not a control. Payload's documentation is explicit:
"The `draft` argument alone does not restrict documents with _status:
'draft' from being returned by the API." A direct GET /cms-api/posts
would have handed every unpublished draft to any visitor. Read access
now returns a query constraint for anonymous callers, which is the
documented mechanism.

The --drop claim was asserted across four files while the spec still
listed it as an open question. Now verified rather than assumed:
run_full_migration drops exactly ["school_results", "schools"] by name,
there is no drop_all() or DROP SCHEMA anywhere in backend/, the only
other drop is schema-qualified to marts, and nothing sets search_path.
The guarantee is stronger than schema isolation alone — those two table
names do not exist in Payload — so the claim stands, but it now rests on
cited code. The spec records the evidence and closes the open item.

findPost is wrapped in React's cache(): Next calls generateMetadata and
the page separately for one request, so every post view ran the same
query against Postgres twice.

The bare .lede rule was dead — .prose p scores (0,1,1) and outranks it —
so only .prose .lede ever applied. Removed, with the specificity noted
so the surviving selector is not "simplified" back into a silent
regression.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017YmbBhr8s7GusjDE12hrZM

nextjs-app/collections/Posts.ts

@@ -24,7 +24,24 @@ function revalidatePost(slug: string) {
 
 export const Posts: CollectionConfig = {
   slug: 'posts',
-  access: { read: () => true },
+  access: {
+    /*
+     * Drafts must be hidden here, not in the pages that query this collection.
+     *
+     * From Payload's own documentation: "The `draft` argument alone does not
+     * restrict documents with `_status: 'draft'` from being returned by the
+     * API." The blog index and post page both filter on `_status`, but that
+     * is a convenience, not a control — a direct GET /cms-api/posts would
+     * hand every unpublished draft to any visitor.
+     *
+     * Returning a query constraint rather than a boolean is the documented
+     * mechanism: Payload merges it into every read for an anonymous caller.
+     */
+    read: ({ req }) => {
+      if (req.user) return true;
+      return { _status: { equals: 'published' } };
+    },
+  },
   admin: {
     useAsTitle: 'title',
     defaultColumns: ['title', 'publishedAt', '_status'],