feat: add BetterAuth authentication

- Add better-auth to backend and frontend
- Create auth tables (users, sessions, accounts, verifications)
- Mount BetterAuth handler on /api/auth/*
- Protect GET /api/tasks with session auth
- Add login page with email/password
- Add invite route for creating users
- Add logout button to header
- Cross-subdomain cookies for .donovankelly.xyz
- Fix page title to 'Hammer Queue'
- Keep bearer token for admin mutations (separate from session auth)
- Update docker-compose with BETTER_AUTH_SECRET and COOKIE_DOMAIN

backend/.env.example

@@ -1,3 +1,6 @@
 DATABASE_URL=postgres://hammer_queue:hammer_queue@localhost:5432/hammer_queue
 API_BEARER_TOKEN=change-me-to-a-secure-token
+BETTER_AUTH_SECRET=change-me-to-a-random-secret
+BETTER_AUTH_URL=https://queue.donovankelly.xyz
+COOKIE_DOMAIN=.donovankelly.xyz
 PORT=3100