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>

src/editor.ts

@@ -1,7 +1,7 @@
 import { spawn, spawnSync } from 'node:child_process'
 import { promises as fs } from 'node:fs'
 import path from 'node:path'
-import { videoDir, loadVideoMeta, saveVideoMeta, type VideoTrim } from './store.js'
+import { getVideo, updateVideo, videoDiskDir, type VideoTrim } from './storeDb.js'
 
 export class FfmpegUnavailableError extends Error {
   constructor() {
@@ -184,14 +184,13 @@ export async function upscaleOriginalTo60Fps(
  * stream copy 를 우선 시도해 빠르게 자르고, 실패하면 재인코딩.
  */
 export async function applyTrimToVideo(
-  folder: string,
   videoId: string,
   trim: VideoTrim
 ): Promise<string> {
   const bin = getFfmpegPath()
-  const meta = await loadVideoMeta(folder, videoId)
+  const meta = getVideo(videoId)
   if (!meta) throw new Error('비디오를 찾을 수 없습니다.')
-  const dir = videoDir(folder, videoId)
+  const dir = videoDiskDir(videoId)
   const inputPath = path.join(dir, meta.originalFile)
   await fs.access(inputPath)
 
@@ -236,9 +235,10 @@ export async function applyTrimToVideo(
   }
   await fs.rename(tmpPath, outPath)
 
-  meta.editedFile = outName
-  meta.trim = { startSec, endSec }
-  await saveVideoMeta(folder, meta)
+  updateVideo(videoId, {
+    editedFile: outName,
+    trim: { startSec, endSec }
+  })
   return outName
 }