Add anti-bot registration protection

- Custom /api/register route: honeypot, disposable email blocking, Cloudflare Turnstile verification
- Register form: honeypot hidden field, Turnstile widget (loads when NEXT_PUBLIC_TURNSTILE_SITE_KEY set)
- Install disposable-email-domains package
- Add NEXT_PUBLIC_TURNSTILE_SITE_KEY and TURNSTILE_SECRET_KEY env vars to .env.example

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -33,6 +33,7 @@
     "better-auth": "^1.6.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
+    "disposable-email-domains": "^1.0.62",
     "gray-matter": "^4.0.3",
     "iconv-lite": "^0.7.2",
     "lucide-react": "^1.7.0",