fix: upgrade Docker to Node 22, add vote persistence diagnostics and e2e tests

- Bump Dockerfile base image from node:18-alpine to node:22-alpine to
  fix build failure (better-sqlite3@12.8.0 requires Node 20+)
- Add post-transaction verification logging in POST /api/votes/live to
  detect live_votes insertion failures in production
- Add direct DB assertion to regression test for live_votes population
- Add end-to-end integration tests covering the full vote flow: POST
  vote -> GET /api/sessions/:id/votes -> GET /api/votes -> direct DB

Made-with: Cursor

tests/api/regression-votes-live.test.js

@@ -1,6 +1,6 @@
 const request = require('supertest');
 const { app } = require('../../backend/server');
-const { getAuthHeader, cleanDb, seedGame, seedSession, seedSessionGame } = require('../helpers/test-utils');
+const { getAuthHeader, cleanDb, seedGame, seedSession, seedSessionGame, db } = require('../helpers/test-utils');
 
 describe('POST /api/votes/live (regression)', () => {
   let game, session, sessionGame;
@@ -46,6 +46,12 @@ describe('POST /api/votes/live (regression)', () => {
       type: 'up',
       timestamp: '2026-03-15T20:05:00.000Z',
     });
+
+    const voteRow = db.prepare('SELECT * FROM live_votes WHERE username = ?').get('viewer1');
+    expect(voteRow).toBeDefined();
+    expect(voteRow.session_id).toBe(session.id);
+    expect(voteRow.game_id).toBe(game.id);
+    expect(voteRow.vote_type).toBe(1);
   });
 
   test('increments downvotes and decrements popularity_score for downvote', async () => {