feat: question-count tiers, footer in main, pregunta footer

- Pricing: 0,99€/10q · 9,99€/200q · 19,99€/unlimited (all 30 days)
- DB: max_questions + questions_used columns on credits table;
  consumeCreditQuestion() atomically validates, creates question,
  and decrements quota in a single transaction
- Server: updated CREDIT_TIERS, /api/credito/estado returns questionsLeft,
  /api/pregunta/enviar returns updated questionsLeft after each submission
- pregunta.tsx: badge shows live question count; submit disabled when exhausted;
  questionsLeft synced to localStorage after each submission; Cloud Host footer added
- footer: moved from Standings sidebar into <main> (scrolls with game content);
  also added to all pregunta page states

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

frontend.tsx

@@ -529,20 +529,6 @@ function Standings({
       {Object.keys(playerScores).length > 0 && (
         <PlayerLeaderboard scores={playerScores} />
       )}
-      <div className="standings__footer">
-        <p>IAs compiten respondiendo preguntas absurdas — los jueces votan, tú también puedes.</p>
-        <p>
-          por{" "}
-          <a
-            href="https://cloudhost.es"
-            target="_blank"
-            rel="noopener noreferrer"
-          >
-            Cloud Host
-          </a>
-          {" "}— La web simplificada, la nube gestionada
-        </p>
-      </div>
     </aside>
   );
 }
@@ -720,6 +706,18 @@ function App() {
               <Dots />
             </div>
           )}
+
+          <footer className="site-footer">
+            <p>IAs compiten respondiendo preguntas absurdas — los jueces votan, tú también puedes.</p>
+            <p>
+              por{" "}
+              <a href="https://cloudhost.es" target="_blank" rel="noopener noreferrer">
+                Cloud Host
+              </a>
+              {" "}— La web simplificada, la nube gestionada ·{" "}
+              <a href="/pregunta">propón preguntas</a>
+            </p>
+          </footer>
         </main>
 
         <Standings