chore(infra): add first-run setup script for new contributors

[GRACENOBLE]

Problem

A new developer cloning this repo must manually: copy .env.example files, configure Firebase credentials, install tooling (Go, pnpm, Android SDK, Docker), and run migrations. None of this is scripted.

Acceptance criteria

• Add a setup.sh (or Makefile setup target) at the repo root that:
  • Checks required tools are installed (go, pnpm, docker, java)
  • Copies .env.example → .env for backend and web if .env doesn't exist yet
  • Installs Node dependencies (pnpm install)
  • Pulls Docker images (docker compose pull)
  • Runs database migrations (make migrate-up)
• Document the script in the root README.md as the first step after cloning

[GRACENOBLE]

Implemented and merged via PR #66.