feat: add doctrine-fixtures-bundle & phpstan, restructure entity mapping

- Add doctrine/doctrine-fixtures-bundle (^4.3) for database fixtures
- Add phpstan/phpstan (^2.0) as dev dependency for static analysis
- Register DoctrineFixturesBundle in dev/test environments
- Move entity mapping to src/Data/Doctrine/Entity/Saatgut
- Update entity prefix to App\Data\Doctrine\Entity\Saatgut
- Change entity alias from App to Saatgut

Backend/config/bundles.php

@@ -5,4 +5,5 @@ return [
     Doctrine\Bundle\DoctrineBundle\DoctrineBundle::class => ['all' => true],
     Symfony\Bundle\SecurityBundle\SecurityBundle::class => ['all' => true],
     Lexik\Bundle\JWTAuthenticationBundle\LexikJWTAuthenticationBundle::class => ['all' => true],
+    Doctrine\Bundle\FixturesBundle\DoctrineFixturesBundle::class => ['dev' => true, 'test' => true],
 ];