feat(folders): nested folder tree (max depth 2) + id-based mutations

- new storeDb.ts: DB+FS layer (folders w/ parent_id, videos w/ global id)
- depth cap: sub-folder cannot have children (enforced in createFolder)
- video id: editable anytime (changeVideoId), uniqueness via PK
- routes:
  - public /folder/:topName[/:subName], /video/:topName[/:subName]/:videoId
  - admin /op/folder/:topName[/:subName], id-based /op/folders/:id/*
    and /op/videos/:id/* (rename/changeId/delete/save + idAvailable check)
- views: breadcrumb + subFolder grid; 플레이리스트 추가 button shows at 2단계
- client JS: id-based mutation endpoints, share URL data attribute,
  in-site player pushes share URL to address bar
- store.ts slimmed to Account + readAccounts (FS layer moved to storeDb)
- pickIntId accepts numeric JSON bodies

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

views/index.ejs

@@ -25,10 +25,10 @@
       <% if (folders.length === 0) { %>
         <p class="muted">아직 폴더가 없습니다. 관리자 페이지에서 폴더를 추가해 주세요.</p>
       <% } %>
-      <% folders.forEach(function (name) { %>
-        <a class="folderCard" href="/folder/<%= encodeURIComponent(name) %>">
+      <% folders.forEach(function (f) { %>
+        <a class="folderCard" href="/folder/<%= encodeURIComponent(f.name) %>">
           <span class="folderIcon">📁</span>
-          <span class="folderName"><%= name %></span>
+          <span class="folderName"><%= f.name %></span>
         </a>
       <% }) %>
     </section>