feat: add .env and database backup patterns to .gitignore

[DiogoRibeiro7]

Proposal

The `.gitignore` is mostly Python/Node focused. Add entries relevant to a MySQL course:

```

Local environment

.env
.secrets/

Database backups

*.sql.bak
*.sql.gz
*.dump

MySQL data files

*.ibd
*.frm
```

Why

Students may create `.env` files for credentials or export database backups into the repo. These should be ignored by default.

Scope / files impacted

• `.gitignore`