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/composer.json

@@ -10,6 +10,7 @@
         "ext-ctype": "*",
         "ext-iconv": "*",
         "doctrine/doctrine-bundle": "*",
+        "doctrine/doctrine-fixtures-bundle": "^4.3",
         "doctrine/orm": "^3.6",
         "lexik/jwt-authentication-bundle": "^3.2",
         "symfony/console": "8.1.*",
@@ -73,5 +74,8 @@
             "allow-contrib": false,
             "require": "8.1.*"
         }
+    },
+    "require-dev": {
+        "phpstan/phpstan": "^2.0"
     }
 }