feat: users answer alongside AI instead of proposing questions

- Core mechanic change: users now submit answers to the live prompt,
  competing alongside AI models; answers broadcast to all viewers
- New pricing (no time limit, pure count-based, refillable):
  0,99€ = 10 resp · 9,99€ = 300 resp · 19,99€ = 1000 resp
- DB: new user_answers table; submitUserAnswer() atomically validates
  credit, inserts answer, decrements budget; JUGADORES leaderboard
  now scores by user_answers count
- server: /api/respuesta/enviar endpoint; credit activation sets
  expires_at 10 years out (effectively no expiry); answers injected
  into live round state and broadcast via WebSocket
- frontend: ProposeAnswer widget shows current prompt, textarea active
  during answering phase, tracks per-round submission state;
  Arena shows Respuestas del público section live

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

game.ts

@@ -67,6 +67,7 @@ export type RoundState = {
   viewerVotesA?: number;
   viewerVotesB?: number;
   viewerVotingEndsAt?: number;
+  userAnswers?: { username: string; text: string }[];
 };
 
 export type GameState = {