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/paths.ts

@@ -13,6 +13,7 @@ export const dataDir = path.join(projectRoot, 'data')
 export const foldersDir = path.join(dataDir, 'folders')
 export const jobsDir = path.join(dataDir, 'jobs')
 export const tmpDir = path.join(dataDir, 'tmp')
+export const dbPath = path.join(dataDir, 'app.db')
 
 export const viewsDir = path.join(projectRoot, 'views')
 export const publicDir = path.join(projectRoot, 'public')