Doctrine ORM Basics

• Entities are plain PHP classes annotated with `#[ORM\Entity]` and `#[ORM\Column]` attributes.
• The `EntityManager` is the central API: `persist()` schedules an insert/update, `remove()` schedules a delete, `flush()` runs queued operations as one transaction.
• Repositories are the read side: `EntityManager::getRepository(User::class)` or inject a custom repository class.
• Doctrine tracks changes automatically — modifying an attached entity then calling `flush()` writes the diff.
• Lifecycle callbacks (`#[ORM\PrePersist]`, `#[ORM\PreUpdate]`) run inside the unit-of-work; keep them simple.

#[ORM\Entity]
class User
{
    #[ORM\Id, ORM\GeneratedValue, ORM\Column]
    private ?int $id = null;

    #[ORM\Column(length: 180, unique: true)]
    private string $email;
}