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>

.gitignore

@@ -18,6 +18,9 @@ __pycache__/
 .env
 .env.local
 .env.*.local
+# But do commit example templates
+!.env.example
+!.env.local.example
 
 # IDE
 .vscode/