관리자 글 목록 고정 기능 추가

db/migrations/064_posts_admin_pinned.sql

@@ -0,0 +1,5 @@
+ALTER TABLE posts
+  ADD COLUMN IF NOT EXISTS admin_pinned BOOLEAN NOT NULL DEFAULT false;
+
+CREATE INDEX IF NOT EXISTS posts_admin_pinned_sort_idx
+  ON posts (admin_pinned DESC, updated_at DESC);