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

frontend/package.json

@@ -11,6 +11,7 @@
   },
   "dependencies": {
     "@tailwindcss/vite": "^4.1.18",
+    "better-auth": "^1.4.17",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "tailwindcss": "^4.1.18"