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>

.env.example

@@ -22,5 +22,6 @@ JWT_REFRESH_EXPIRES_IN=30d
 PODCAST_INDEX_API_KEY=
 PODCAST_INDEX_API_SECRET=
 
-# Frontend Configuration
-VITE_API_URL=http://localhost:5000/api
+# Frontend Configuration (only needed for local dev without Docker)
+# In Docker, the frontend is served by the backend itself — no VITE_API_URL needed
+VITE_API_URL=/api