quiz: split bank into two host-selectable 15-question pools

The competition runs as two back-to-back sittings with different
students, so the bank is now two independent pools of matched shape:
5 astronomy (2 image IDs) + 10 Indian space technology = 15 each.

Pools are host-side only. Players register and wait exactly as before;
the reveal payload carries a per-pool index and a total of 15 either
way, so nothing about pool selection reaches the client. Switching is
refused while a question is open, clears every answer, and drops the
room back to lobby. Mongo snapshots are keyed per pool so the first
sitting's leaderboard survives the second.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: RedRangerWentWild

sast-backend/models/QuizSession.js

@@ -11,6 +11,7 @@ const mongoose = require('mongoose');
 const QuizSessionSchema = new mongoose.Schema(
   {
     sessionId: { type: String, required: true, unique: true, index: true },
+    poolId: { type: Number, default: 1 },
     phase: { type: String, enum: ['lobby', 'live', 'ended'], default: 'lobby' },
     questionCount: Number,
     lastQuestionIndex: Number,

sast-backend/quiz/engine.js

@@ -1,5 +1,5 @@
 const crypto = require('crypto');
-const { QUESTIONS } = require('./questions');
+const { getPool, DEFAULT_POOL } = require('./questions');
 
 /**
  * Live quiz state machine.
@@ -29,8 +29,11 @@ const norm = (s) => String(s ?? '').trim().replace(/\s+/g, ' ');
 const rollKey = (s) => norm(s).toUpperCase();
 
 class QuizEngine {
-  constructor({ questions = QUESTIONS, now = () => Date.now() } = {}) {
-    this.questions = questions;
+  constructor({ questions, pool = DEFAULT_POOL, now = () => Date.now() } = {}) {
+    // An explicit `questions` bank wins (tests use it); otherwise the engine
+    // is backed by one of the two competition pools.
+    this.poolId = questions ? null : Number(pool);
+    this.questions = questions || getPool(pool);
     this.now = now;
     this.reset();
   }
@@ -47,6 +50,35 @@ class QuizEngine {
     return this.questions.length;
   }
 
+  /**
+   * Load one of the competition pools.
+   *
+   * Refused while a question is open — swapping the bank out from under a
+   * live round would invalidate the answers already in flight. Registered
+   * players are kept (the host may have a room already waiting) but their
+   * answers are dropped, since indices mean something different in the new
+   * pool. Use reset() to clear the roster too.
+   */
+  setPool(id) {
+    if (this.round && !this.round.closed) {
+      return { error: 'Close the current question before switching pools.' };
+    }
+    const next = Number(id);
+    const questions = getPool(next);
+    this.poolId = next;
+    this.questions = questions;
+
+    this.phase = 'lobby';
+    this.round = null;
+    this.startedAt = null;
+    this.endedAt = null;
+    for (const p of this.players.values()) {
+      p.answers.clear();
+      p.ready = false;
+    }
+    return { poolId: this.poolId, total: this.total };
+  }
+
   /* ── players ─────────────────────────────────────────────── */
 
   join({ name, roll }) {
@@ -274,6 +306,7 @@ class QuizEngine {
     }
     return {
       phase: this.phase,
+      poolId: this.poolId,
       total: this.total,
       players: this.players.size,
       connected,