fix: add createdAt index to ReviewCard model to fix slow admin page load

The admin review list query uses ORDER BY createdAt DESC but there was no
index on createdAt, causing full table scan + filesort on large tables.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

prisma/schema.prisma

@@ -521,6 +521,7 @@ model ReviewCard {
   @@index([userId])
   @@index([nextReviewAt])
   @@index([focusItemId])
+  @@index([createdAt])
 }
 
 model ReviewLog {