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>

public/public-folder.js

@@ -1,7 +1,7 @@
 (function () {
   var ctxMenu = document.getElementById('ctxMenu')
   if (!ctxMenu) return
-  var targetId = null
+  var targetShareUrl = null
 
   function showCtx(x, y) {
     ctxMenu.style.left = x + 'px'
@@ -13,7 +13,7 @@
   document.querySelectorAll('.videoCard').forEach(function (card) {
     card.addEventListener('contextmenu', function (e) {
       e.preventDefault()
-      targetId = card.getAttribute('data-video-id') || card.getAttribute('data-id')
+      targetShareUrl = card.getAttribute('data-share-url')
       showCtx(e.clientX, e.clientY)
     })
   })
@@ -27,22 +27,19 @@
     var btn = e.target.closest('button')
     if (!btn) return
     var action = btn.getAttribute('data-action')
-    if (action === 'copyUrl' && targetId) {
-      copyVideoUrl(targetId)
+    if (action === 'copyUrl' && targetShareUrl) {
+      copyUrl(location.origin + targetShareUrl)
     }
     hideCtx()
   })
 
-  function copyVideoUrl(id) {
-    var url = location.origin + '/file/video/' + encodeURIComponent(id)
+  function copyUrl(text) {
     if (navigator.clipboard && navigator.clipboard.writeText) {
-      navigator.clipboard.writeText(url).then(function () {
+      navigator.clipboard.writeText(text).then(function () {
         flashToast('주소가 복사되었습니다.')
-      }, function () {
-        fallbackCopy(url)
-      })
+      }, function () { fallbackCopy(text) })
     } else {
-      fallbackCopy(url)
+      fallbackCopy(text)
     }
   }
   function fallbackCopy(text) {