feat: footer, auto-pause on no viewers, and Redsys question submissions

- Footer: add site-footer with one-liner + Cloud Host branding linking to cloudhost.es (mobile only, hidden on desktop)
- Auto-pause: game pauses automatically after AUTOPAUSE_DELAY_MS (default 60s) with no viewers connected; auto-resumes when first viewer connects; shows "Esperando espectadores…" in the UI
- Redsys: new /pregunta page lets viewers pay 1€ via Redsys to submit a fill-in question; submitted questions are used as prompts in the next round instead of AI generation; new redsys.ts implements HMAC_SHA256_V1 signing (3DES key derivation + HMAC-SHA256); notification endpoint at /api/redsys/notificacion handles server-to-server confirmation
- db.ts: questions table with createPendingQuestion / markQuestionPaid / getNextPendingQuestion / markQuestionUsed
- .env.sample: added AUTOPAUSE_DELAY_MS, PUBLIC_URL, REDSYS_* vars

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

frontend.css

@@ -736,6 +736,28 @@ body {
 ::-webkit-scrollbar-track { background: transparent; }
 ::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }
 
+/* ── Site Footer ──────────────────────────────────────────────── */
+
+.site-footer {
+  flex-shrink: 0;
+  padding: 14px 20px;
+  border-top: 1px solid var(--border);
+  text-align: center;
+  font-size: 12px;
+  color: var(--text-muted);
+  line-height: 1.7;
+}
+
+.site-footer__link {
+  color: var(--text-dim);
+  text-decoration: none;
+}
+
+.site-footer__link:hover {
+  color: var(--text);
+  text-decoration: underline;
+}
+
 /* ── Desktop (1024px+) ───────────────────────────────────────── */
 
 @media (min-width: 1024px) {
@@ -791,4 +813,6 @@ body {
     padding: 24px;
     gap: 24px;
   }
+
+  .site-footer { display: none; }
 }