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/Entity/Composant.php

@@ -16,6 +16,7 @@ use ApiPlatform\Metadata\Post;
 use ApiPlatform\Metadata\Put;
 use App\Entity\Trait\CuidEntityTrait;
 use App\Repository\ComposantRepository;
+use App\State\ComposantProcessor;
 use DateTimeImmutable;
 use Doctrine\Common\Collections\ArrayCollection;
 use Doctrine\Common\Collections\Collection;
@@ -33,7 +34,7 @@ use Symfony\Component\Serializer\Attribute\Groups;
     operations: [
         new Get(security: "is_granted('ROLE_VIEWER')"),
         new GetCollection(security: "is_granted('ROLE_VIEWER')"),
-        new Post(security: "is_granted('ROLE_GESTIONNAIRE')"),
+        new Post(security: "is_granted('ROLE_GESTIONNAIRE')", processor: ComposantProcessor::class),
         new Put(security: "is_granted('ROLE_GESTIONNAIRE')"),
         new Patch(security: "is_granted('ROLE_GESTIONNAIRE')"),
         new Delete(security: "is_granted('ROLE_GESTIONNAIRE')"),