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/symfony.lock

@@ -22,6 +22,18 @@
             "src/Repository/.gitignore"
         ]
     },
+    "doctrine/doctrine-fixtures-bundle": {
+        "version": "4.3",
+        "recipe": {
+            "repo": "github.com/symfony/recipes",
+            "branch": "main",
+            "version": "3.0",
+            "ref": "1f5514cfa15b947298df4d771e694e578d4c204d"
+        },
+        "files": [
+            "src/DataFixtures/AppFixtures.php"
+        ]
+    },
     "lexik/jwt-authentication-bundle": {
         "version": "3.2",
         "recipe": {
@@ -34,6 +46,9 @@
             "config/packages/lexik_jwt_authentication.yaml"
         ]
     },
+    "phpstan/phpstan": {
+        "version": "2.2.2"
+    },
     "symfony/console": {
         "version": "8.1",
         "recipe": {