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-lock.json

@@ -28,6 +28,7 @@
         "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",
@@ -6302,6 +6303,12 @@
         "node": ">=0.3.1"
       }
     },
+    "node_modules/disposable-email-domains": {
+      "version": "1.0.62",
+      "resolved": "https://registry.npmjs.org/disposable-email-domains/-/disposable-email-domains-1.0.62.tgz",
+      "integrity": "sha512-LBQvhRw7mznQTPoyZbsmYeNOZt1pN5aCsx4BAU/3siVFuiM9f2oyKzUaB8v1jbxFjE3aYqYiMo63kAL4pHgfWQ==",
+      "license": "MIT"
+    },
     "node_modules/doctrine": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/doctrine/-/doctrine-2.1.0.tgz",