fix: snapshot payout terms at game start (what-you-saw-is-what-you-get)

Payouts were computed at completion from LIVE config, so an admin changing
multipliers mid-game would change the payout of an already-in-progress game.

- Migration 016 adds payout_escape_amount/payout_treasure_amount (BIGINT) and
  the corresponding multipliers to the games table.
- Both game-start paths stamp the resolved payout amounts (computed from the
  config in effect at entry) onto the game row.
- completeGame pays from the snapshot, falling back to live calculation only
  for legacy games that predate the snapshot columns.

Adds payoutSnapshot.test.js proving a mid-game config change cannot alter an
in-flight payout. Full suite: 15 suites / 131 tests.

test/payoutSnapshot.test.js

@@ -0,0 +1,92 @@
+/**
+ * Payout snapshot tests (Phase 1.2).
+ *
+ * The payout terms are snapshotted onto the game at start. completeGame must pay from
+ * that snapshot, so a mid-game config change can't alter an in-flight game's payout.
+ */
+
+const GameModeManager = require('../src/game/gameModeManager');
+
+const createMockDb = () => {
+  const mockClient = { query: jest.fn().mockResolvedValue({ rows: [] }) };
+  return {
+    query: jest.fn().mockResolvedValue({ rows: [] }),
+    withTransaction: jest.fn().mockImplementation(async (cb) => cb(mockClient)),
+    _mockClient: mockClient
+  };
+};
+
+const createMockPaymentConfig = () => ({
+  getConfig: () => ({
+    paymentsEnabled: true,
+    currency: { symbol: 'WOW', decimals: 11 },
+    modes: {
+      direct: { enabled: true, price: 100000000000n },
+      credits: { enabled: true, creditsPerGame: 1, packages: [{ id: 'small', credits: 10, price: '500000000000', bonus: 0 }] }
+    },
+    payouts: {
+      rules: {
+        direct: { enabled: true, multipliers: { escape: 2, escapeWithTreasure: 3 } },
+        credits: { enabled: false, multipliers: { escape: 1.5, escapeWithTreasure: 2 }, baseValue: 50000000000n }
+      }
+    },
+    preferences: { preferCreditsFirst: true },
+    earlyEntry: { enabled: false }
+  }),
+  getLegacyGameMode: () => 'PAID_SINGLE',
+  eventBus: { on: () => {} }
+});
+
+function buildGmm() {
+  const db = createMockDb();
+  const gmm = new GameModeManager(
+    db,
+    { processPayout: jest.fn(), processBatchPayout: jest.fn() },
+    { CONSOLE_LOGGING: false, getCurrentBlockHeight: () => 1 },
+    createMockPaymentConfig()
+  );
+  gmm._scheduleBatchPayout = jest.fn(); // don't actually schedule
+  return { gmm, db };
+}
+
+describe('Payout uses the snapshot recorded at game start', () => {
+  test('pays the snapshot amount, not the live-config amount', async () => {
+    const { gmm, db } = buildGmm();
+
+    // Snapshot recorded at start: escape pays 123456 atomic (deliberately unlike the
+    // live 2x = 200000000000), treasure pays 999999.
+    db.query
+      .mockResolvedValueOnce({ rows: [{
+        id: 1, game_mode: 'PAID_SINGLE', payout_address: 'wow1addr',
+        payout_escape_amount: '123456', payout_treasure_amount: '999999',
+        payout_escape_mult: '2.000', payout_treasure_mult: '3.000'
+      }] })                                   // game record lookup
+      .mockResolvedValueOnce({ rows: [] })     // game completion UPDATE
+      .mockResolvedValueOnce({ rows: [] })     // existing-payout check
+      .mockResolvedValueOnce({ rows: [{ id: 1, payout_address: 'wow1addr' }] }) // user lookup
+      .mockResolvedValueOnce({ rows: [{ id: 999 }] }); // INSERT payout
+
+    // Even if live config is mutated after start, the snapshot must win.
+    gmm.directPayoutMultipliers = { escape: 9, escapeWithTreasure: 9 };
+
+    const result = await gmm.completeGame('socket1', 'game123', true, false, { moves: 10 });
+
+    expect(result.payout).toBeDefined();
+    expect(result.payout.amount).toBe('123456');
+  });
+
+  test('falls back to live calculation for legacy games without a snapshot', async () => {
+    const { gmm, db } = buildGmm();
+
+    db.query
+      .mockResolvedValueOnce({ rows: [{ id: 1, game_mode: 'PAID_SINGLE', payout_address: 'wow1addr' }] }) // no snapshot cols
+      .mockResolvedValueOnce({ rows: [] })
+      .mockResolvedValueOnce({ rows: [] })
+      .mockResolvedValueOnce({ rows: [{ id: 1, payout_address: 'wow1addr' }] })
+      .mockResolvedValueOnce({ rows: [{ id: 999 }] });
+
+    const result = await gmm.completeGame('socket1', 'game123', true, false, { moves: 10 });
+
+    expect(result.payout.amount).toBe(200000000000); // live 2x
+  });
+});