feat(directory) : add Prospect entity with conversion to Client (back)

LST-58 (2.4), part 2 — Prospect (new entity). Completes the Directory backend.

- ProspectStatus enum (new/contacted/qualified/won/lost) + Prospect entity
  (name, company, email, phone, address, status, source, notes,
  convertedClient -> ClientInterface) with Timestampable/Blamable + #[Auditable].
- API: GetCollection/Get (ROLE_USER), Post/Patch/Delete (ROLE_ADMIN),
  custom POST /prospects/{id}/convert (ConvertProspectProcessor: creates a
  Client from the prospect, links convertedClient, sets status=Won; idempotent).
  SearchFilter on status.
- Repository interface + Doctrine impl (bound); 6 MCP tools (list/get/create/
  update/delete/convert-prospect); Serializer::prospect(). Module perms
  directory.prospects.view/manage. Demo fixtures (3 prospects, one converted).
- Additive migration: CREATE TABLE prospect + FKs ON DELETE SET NULL + COMMENT.

163 tests green (incl. conversion test), mapping valid, cs-fixer clean.

src/Module/Directory/DirectoryModule.php

@@ -36,6 +36,8 @@ final class DirectoryModule implements ModuleInterface
         return [
             ['code' => 'directory.clients.view', 'label' => 'Voir les clients'],
             ['code' => 'directory.clients.manage', 'label' => 'Gérer les clients'],
+            ['code' => 'directory.prospects.view', 'label' => 'Voir les prospects'],
+            ['code' => 'directory.prospects.manage', 'label' => 'Gérer les prospects'],
         ];
     }
 }