Implements P0-004: authentication package with BetterAuth + Drizzle adapter.
- BetterAuth configured with Drizzle adapter (usePlural for table mapping)
- Email/password authentication enabled
- Role field (admin/member/viewer) as additional user field
- 7-day sessions with daily refresh
- Mounted at /api/auth/* basePath
- createAuth() factory takes Db instance for DI flexibility
- BETTER_AUTH_SECRET and BETTER_AUTH_URL env vars added to .env.example
Verified: auth instance created, handler and API methods confirmed.
Closes#4
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
