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/phpstan.neon

@@ -0,0 +1,7 @@
+parameters:
+    level: 8
+    paths:
+        - src
+    ignoreErrors:
+        # Symfony Kernel has some generic unused methods that are technically framework internals.
+        - '#Method App\\Kernel::getAllowedEnvs\(\) is unused#'