Add unique constraint for definition_id and person_id in PersonIdentifier

- Update entity to include the new database-level unique constraint.
- Add migration script to apply the unique constraint and handle rollback.

src/Bundle/ChillPersonBundle/Entity/Identifier/PersonIdentifier.php

@@ -18,6 +18,7 @@ use Doctrine\ORM\Mapping as ORM;
 #[ORM\Entity]
 #[ORM\Table(name: 'chill_person_identifier')]
 #[ORM\UniqueConstraint(name: 'chill_person_identifier_unique', columns: ['definition_id', 'canonical'])]
+#[ORM\UniqueConstraint(name: 'chill_person_identifier_unique_person_definition', columns: ['definition_id', 'person_id'])]
 #[UniqueIdentifierConstraint]
 class PersonIdentifier
 {