fix: standardize Old Book → Old Books folder name

- filebrowserFolders.js: createTaskFolder now creates Old Books
- fbq-backfill: creates Old Books; migrateOldBook() merges/renames
  existing Old Book folders into Old Books on each backfill run

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

supabase/functions/fbq-backfill/index.ts

@@ -32,6 +32,45 @@ function mime(name: string) {
   return m[e] ?? 'application/octet-stream';
 }
 
+async function listDir(path: string): Promise<{ name: string; type: string }[] | null> {
+  const r = await fetch(`${FBQ_URL}/api/resources?source=${SOURCE}&path=${encodeURIComponent(path)}`, { headers: fbqH });
+  if (!r.ok) return null;
+  const data = await r.json().catch(() => null);
+  return [...(data?.folders ?? []), ...(data?.files ?? [])];
+}
+
+async function renameDir(fromPath: string, toPath: string) {
+  await fetch(`${FBQ_URL}/api/resources?source=${SOURCE}`, {
+    method: 'PATCH',
+    headers: { ...fbqH, 'Content-Type': 'application/json' },
+    body: JSON.stringify({ action: 'rename', items: [{ fromSource: SOURCE, fromPath, toSource: SOURCE, toPath }], overwrite: false }),
+  });
+}
+
+async function deleteDir(path: string) {
+  await fetch(`${FBQ_URL}/api/resources?source=${SOURCE}&path=${encodeURIComponent(path.endsWith('/') ? path : path + '/')}`, {
+    method: 'DELETE', headers: fbqH,
+  });
+}
+
+async function migrateOldBook(base: string) {
+  const src = `${base}/Old Book`;
+  const dst = `${base}/Old Books`;
+  const srcItems = await listDir(src);
+  if (srcItems === null) return; // Old Book doesn't exist
+  const dstItems = await listDir(dst);
+  if (dstItems === null) {
+    // Old Books doesn't exist — simple rename
+    await renameDir(src, dst);
+  } else {
+    // Both exist — move each item from Old Book into Old Books, then delete Old Book
+    for (const item of srcItems) {
+      await renameDir(`${src}/${item.name}`, `${dst}/${item.name}`);
+    }
+    await deleteDir(src);
+  }
+}
+
 async function chunkAll<T>(items: T[], fn: (item: T) => Promise<void>, size = 8) {
   for (let i = 0; i < items.length; i += size) {
     await Promise.all(items.slice(i, i + size).map(fn));
@@ -90,7 +129,8 @@ Deno.serve(async () => {
     ]);
 
     const taskIsNew = await mkdir(base);
-    await Promise.all([mkdir(`${base}/Request Info`), mkdir(`${base}/Old Book`)]);
+    await Promise.all([mkdir(`${base}/Request Info`), mkdir(`${base}/Old Books`)]);
+    await migrateOldBook(base);
 
     if (taskIsNew) { newTasks.push({ task, base, co, pr }); created++; }
     else skipped++;