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

backend/routes/votes.js

@@ -231,6 +231,16 @@ router.post('/live', authenticateToken, (req, res) => {
 
     processVote();
 
+    // Verify the live_votes row was persisted (diagnostic for production debugging)
+    const voteCheck = db.prepare(
+      'SELECT id FROM live_votes WHERE session_id = ? AND game_id = ? AND username = ? AND timestamp = ?'
+    ).get(activeSession.id, matchedGame.game_id, username, timestamp);
+    if (!voteCheck) {
+      console.error('[votes] CRITICAL: live_votes INSERT committed but row not found', {
+        session_id: activeSession.id, game_id: matchedGame.game_id, username, timestamp,
+      });
+    }
+
     // Get updated game stats
     const updatedGame = db.prepare(`
       SELECT id, title, upvotes, downvotes, popularity_score