Fix Docker Compose frontend-to-backend networking

[Copilot]

The frontend container was configured to connect to localhost:8000, which resolves to itself rather than the backend service in Docker Compose networking.

Changes:

• Set BACKEND_URL=http://backend:8000 environment variable in frontend service to use Docker's internal DNS resolution

frontend:
  build: ./frontend
  ports:
    - "5173:5173"
  environment:
    - BACKEND_URL=http://backend:8000  # Added
  volumes:
    - ./frontend:/app
    - /app/node_modules
  depends_on:
    - backend

The vite.config.js already reads process.env.BACKEND_URL, so no code changes were required.

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