fix(composant) : scaffold skeleton slots on creation + explicit unique constraint errors

- Add ComposantProcessor: initializes piece/product/subcomponent slots
  from ModelType skeleton requirements when a composant is created
- UniqueConstraintSubscriber: priority 256, French error messages,
  constraint name detection for specific feedback
- Migration: scaffold missing slots for existing composants in prod

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

src/EventSubscriber/UniqueConstraintSubscriber.php

@@ -16,7 +16,7 @@ final class UniqueConstraintSubscriber implements EventSubscriberInterface
     public static function getSubscribedEvents(): array
     {
         return [
-            KernelEvents::EXCEPTION => 'onKernelException',
+            KernelEvents::EXCEPTION => ['onKernelException', 256],
         ];
     }
 
@@ -28,10 +28,17 @@ final class UniqueConstraintSubscriber implements EventSubscriberInterface
             return;
         }
 
+        $constraint = $this->detectConstraintName($exception);
+        $error      = match ($constraint) {
+            'unique_category_name' => 'Un élément avec ce nom existe déjà dans cette catégorie.',
+            default                => 'Un élément avec cette valeur existe déjà.',
+        };
+
         $event->setResponse(new JsonResponse(
             [
-                'success' => false,
-                'error'   => 'nom duplique',
+                'success'    => false,
+                'error'      => $error,
+                'constraint' => $constraint,
             ],
             JsonResponse::HTTP_CONFLICT
         ));
@@ -47,4 +54,15 @@ final class UniqueConstraintSubscriber implements EventSubscriberInterface
 
         return null;
     }
+
+    private function detectConstraintName(UniqueConstraintViolationException $exception): ?string
+    {
+        $message = $exception->getMessage();
+
+        if (preg_match('/constraint\s+"([^"]+)"/', $message, $matches)) {
+            return $matches[1];
+        }
+
+        return null;
+    }
 }