Add setup and deployment guide, env example files

- Rewrote README.md with full step-by-step setup guide covering Docker
  Compose deployment, environment configuration, port reference, local
  dev setup, and reset instructions
- Added backend/.env.example and frontend/.env.local.example so new
  deployers know exactly what to configure without any secrets committed
- Updated .gitignore to allow .env.example files through

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/.gitignore

@@ -32,6 +32,7 @@ yarn-error.log*
 
 # env files (can opt-in for committing if needed)
 .env*
+!.env.local.example
 
 # vercel
 .vercel