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>

frontend.css

@@ -769,7 +769,53 @@ body {
   white-space: nowrap;
 }
 
-/* ── Propose Question widget ─────────────────────────────────── */
+/* ── User answers (audience) ─────────────────────────────────── */
+
+.user-answers {
+  margin-top: 20px;
+  padding-top: 16px;
+  border-top: 1px solid var(--border);
+}
+
+.user-answers__label {
+  font-family: var(--mono);
+  font-size: 10px;
+  font-weight: 700;
+  letter-spacing: 1.5px;
+  text-transform: uppercase;
+  color: var(--text-muted);
+  margin-bottom: 8px;
+}
+
+.user-answers__list {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+}
+
+.user-answer {
+  font-size: 13px;
+  line-height: 1.4;
+}
+
+.user-answer__name {
+  font-family: var(--mono);
+  font-size: 11px;
+  font-weight: 700;
+  color: var(--accent);
+}
+
+.user-answer__sep {
+  color: var(--text-muted);
+}
+
+.user-answer__text {
+  color: var(--text-dim);
+  font-family: var(--serif);
+  font-size: 15px;
+}
+
+/* ── Propose Answer widget ───────────────────────────────────── */
 
 .propose {
   flex-shrink: 0;