fix(data-integrity) : prevent data loss in clone, slots, conversion and custom fields

- Clone: CustomFieldValue now references cloned CustomField, not source
- Slots: validate piece type matches slot requirement + 404 on missing piece
- Conversion: check slot tables before allowing category conversion + clean orphan skeleton requirements
- CustomFieldValue: prevent creation of orphan CustomField without target entity

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Controller/CustomFieldValueController.php

@@ -196,19 +196,16 @@ class CustomFieldValueController extends AbstractController
             return $this->json(['success' => false, 'error' => 'customFieldId or customFieldName is required.'], 400);
         }
 
-        $customField = new CustomField();
-        $customField->setName($customFieldName);
-        $customField->setType((string) ($payload['customFieldType'] ?? 'text'));
-        $customField->setRequired((bool) ($payload['customFieldRequired'] ?? false));
-
-        $options = $payload['customFieldOptions'] ?? null;
-        if (is_array($options)) {
-            $customField->setOptions($options);
+        // Try to find an existing custom field by name instead of creating an orphan
+        $existing = $this->customFieldRepository->findOneBy(['name' => $customFieldName]);
+        if ($existing instanceof CustomField) {
+            return $existing;
         }
 
-        $this->entityManager->persist($customField);
-
-        return $customField;
+        return $this->json([
+            'success' => false,
+            'error'   => sprintf('Custom field "%s" not found. Create it explicitly first.', $customFieldName),
+        ], 404);
     }
 
     private function resolveTarget(array $payload): array|JsonResponse