P1: Add docker-compose local stack (mysql + php) + env templates

[Kinin-Code-Offical]

Local development setup is inconsistent. A Docker-based environment will ensure all developers have the same setup.

Acceptance Criteria:

• Create a docker-compose.yml file at the root.
• The stack should include a mysql:8 service.
• The stack should include a php-apache service with the necessary extensions (mysqli, pdo_mysql).
• The PHP service should mount the �ackend directory.
• Create an .env.example file for environment variables (MYSQL_ROOT_PASSWORD, MYSQL_DATABASE, etc.).
• Update README.md with instructions on how to use Docker for local development.