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/editor.js

@@ -1,6 +1,7 @@
 (function () {
-  var ctx = window.__EDITOR__ || { folder: '', video: null }
-  var folder = ctx.folder
+  var ctx = window.__EDITOR__ || { folderPath: [], folderId: null, video: null }
+  var folderPathEnc = (ctx.folderPath || []).map(encodeURIComponent).join('/')
+  var folderBaseUrl = '/op/folder/' + folderPathEnc
   var video = ctx.video
 
   var dropZone = document.getElementById('dropZone')
@@ -46,7 +47,7 @@
     form.append('title', titleInput.value || file.name)
     uploadStatus.textContent = '업로드 중...'
     var xhr = new XMLHttpRequest()
-    xhr.open('POST', '/op/folder/' + encodeURIComponent(folder) + '/video/upload')
+    xhr.open('POST', folderBaseUrl + '/video/upload')
     xhr.upload.addEventListener('progress', function (e) {
       if (e.lengthComputable) {
         var pct = Math.round((e.loaded / e.total) * 100)
@@ -57,7 +58,7 @@
       try {
         var res = JSON.parse(xhr.responseText)
         if (res.ok) {
-          location.href = '/op/folder/' + encodeURIComponent(folder) + '/video/editor?id=' + encodeURIComponent(res.videoId)
+          location.href = folderBaseUrl + '/video/editor?id=' + encodeURIComponent(res.videoId)
         } else {
           uploadStatus.textContent = '업로드 실패: ' + (res.message || '')
         }
@@ -89,7 +90,7 @@
     probeInfo.textContent = '확인 중...'
     ytProbeBtn.disabled = true
     ytStartBtn.disabled = true
-    fetch('/op/folder/' + encodeURIComponent(folder) + '/video/youtube/probe', {
+    fetch(folderBaseUrl + '/video/youtube/probe', {
       method: 'POST',
       headers: { 'content-type': 'application/json' },
       body: JSON.stringify({ url: url })
@@ -144,7 +145,7 @@
     }
     // 중복 클릭 방지
     ytStartBtn.disabled = true
-    fetch('/op/folder/' + encodeURIComponent(folder) + '/video/youtube/start', {
+    fetch(folderBaseUrl + '/video/youtube/start', {
       method: 'POST',
       headers: { 'content-type': 'application/json' },
       body: JSON.stringify({ url: url, title: titleInput.value })
@@ -180,7 +181,7 @@
         dlProgress.value = job.progress
         probeInfo.textContent = job.message
         if (job.status === 'done') {
-          location.href = '/op/folder/' + encodeURIComponent(folder) + '/video/editor?id=' + encodeURIComponent(videoId)
+          location.href = folderBaseUrl + '/video/editor?id=' + encodeURIComponent(videoId)
         } else if (job.status === 'error') {
           probeInfo.textContent = '실패: ' + (job.error || '')
         } else {
@@ -364,14 +365,13 @@
   saveBtn.addEventListener('click', function () {
     if (!video) { alert('먼저 영상을 추가하세요.'); return }
     var payload = {
-      id: video.id,
       title: titleInput.value,
       startSec: trimStart,
       endSec: trimEnd
     }
     saveBtn.disabled = true
     saveBtn.textContent = '저장 중...'
-    fetch('/op/folder/' + encodeURIComponent(folder) + '/video/save', {
+    fetch('/op/videos/' + encodeURIComponent(video.id) + '/save', {
       method: 'POST',
       headers: { 'content-type': 'application/json' },
       body: JSON.stringify(payload)
@@ -380,7 +380,7 @@
       saveBtn.textContent = '저장'
       if (j.ok) {
         alert(j.note || '저장 완료')
-        location.href = '/op/folder/' + encodeURIComponent(folder)
+        location.href = folderBaseUrl
       } else {
         alert(j.message || '저장 실패')
       }