Set up data backend project structure (#50)

Add scaffolding for the new SQLite-backed data pipeline:
- scripts/backend/ with CLI entry point, db module, and directory structure
- tsconfig.backend.json targeting Node.js with strict mode
- better-sqlite3 + tsx dependencies
- npm scripts: db:migrate, db:fetch, db:export
- data/*.db gitignored

Closes #22

Co-authored-by: Tasks Agent <tasks@localhost>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>