feat(db): introduce SQLite (better-sqlite3) backing store

First step of the folder/video refactor. Adds an app-local SQLite DB at
data/app.db with schema for nested folders (max 2 levels enforced in
code, not SQL) and user-editable globally-unique video IDs.

- src/db.ts: open + WAL + schema (folders, videos) + partial unique
  index for top-level folder names (NULL parent_id case)
- One-shot disk-tree migration: on first start scans data/folders/*
  and seeds folder rows; reads each {videoId}/meta.json to seed video
  rows. Skips invalid/missing meta gracefully.
- Wired into app.ts startup before routes mount.
- gitignore: data/app.db* (DB + WAL/journal sidecars).

Schema only — no routes or store.ts changes yet. Next step swaps the
file-based store calls to DB-backed equivalents.

Smoke-tested on the current data dir: 3 folders migrated, 0 videos
(folders were empty). Build (npx tsc) passes.

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

src/app.ts

@@ -6,6 +6,7 @@ import fsp from 'node:fs/promises'
 import { dataDir, foldersDir, jobsDir, tmpDir, publicDir, viewsDir } from './paths.js'
 import { publicRouter } from './routes/public.js'
 import { opRouter } from './routes/op.js'
+import { initDatabase } from './db.js'
 
 async function ensureDirs(): Promise<void> {
   for (const dir of [dataDir, foldersDir, jobsDir, tmpDir]) {
@@ -15,6 +16,8 @@ async function ensureDirs(): Promise<void> {
 
 async function main(): Promise<void> {
   await ensureDirs()
+  // SQLite 스키마 보장 + 비어 있으면 기존 디스크 트리에서 마이그레이션.
+  await initDatabase()
 
   const PORT = Number(process.env.PORT ?? 3000)
   const HOST = process.env.HOST ?? '127.0.0.1'