fix: single container without nginx — Express serves frontend

- Remove separate backend/ and frontend/ Dockerfiles
- Add root Dockerfile (3-stage: frontend build → backend build → runtime)
- Frontend static files served by Express from /public
- Simplify docker-compose to 3 services: mongodb, redis, app (port 3579)
- Fix vite proxy not to strip /api prefix from paths
- Default API URL to relative /api instead of hardcoded localhost

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/services/api.ts

@@ -1,6 +1,6 @@
 import axios from 'axios';
 
-const API_URL = import.meta.env.VITE_API_URL || 'http://localhost:5000/api';
+const API_URL = import.meta.env.VITE_API_URL || '/api';
 
 const api = axios.create({
   baseURL: API_URL,