feat: add .env support and document default credentials

[DiogoRibeiro7]

Proposal

Scripts default to `root` / `root` for MySQL credentials (hardcoded in `check_integrity.sh` line 7). This assumption is undocumented and breaks for students who set a different password during MySQL installation.

Changes

1. Add a `.env.example` file with documented defaults:
   ```
   MYSQL_HOST=127.0.0.1
   MYSQL_PORT=3306
   MYSQL_USER=root
   MYSQL_PASSWORD=root
   ```
2. Add `.env` to `.gitignore` (prevent students from committing credentials)
3. Document in setup guide that scripts read from environment variables

Why

Students who follow the setup guide and set a strong password find that scripts don't work. Clear credential documentation prevents this.

Scope / files impacted

• `.env.example` (new)
• `.gitignore`
• `scripts/README.md`
• `course/00_setup/01_workbench_connection.md`