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/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:18-alpine
+FROM node:22-alpine
 
 WORKDIR /app