feat: admin can answer questions without paying for testing

- Server: /api/respuesta/enviar checks admin cookie; if authorized,
  bypasses credit check and stores answer via insertAdminAnswer()
- DB: insertAdminAnswer() inserts directly into user_answers with
  username='Admin', skipping the credit budget entirely
- Frontend: ProposeAnswer checks /api/admin/status on mount; if admin
  is logged in, shows the answer form directly (orange Admin badge)
  instead of the payment tier selection

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

db.ts

@@ -189,6 +189,13 @@ export function getCreditByOrder(orderId: string): {
   } | null;
 }
 
+/** Insert a user answer directly, bypassing credit checks (admin use). */
+export function insertAdminAnswer(roundNum: number, text: string, username: string): void {
+  db.prepare(
+    "INSERT INTO user_answers (round_num, text, username, token) VALUES ($roundNum, $text, $username, 'admin')"
+  ).run({ $roundNum: roundNum, $text: text, $username: username });
+}
+
 /**
  * Atomically validates a credit token, records a user answer for the given
  * round, and decrements the answer budget. Returns null if the token is