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/Domain/Repository/ProspectRepositoryInterface.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Module\Directory\Domain\Repository;
+
+use App\Module\Directory\Domain\Entity\Prospect;
+
+interface ProspectRepositoryInterface
+{
+    public function findById(int $id): ?Prospect;
+
+    /**
+     * @param array<string, mixed>       $criteria
+     * @param null|array<string, string> $orderBy
+     *
+     * @return Prospect[]
+     */
+    public function findBy(array $criteria, ?array $orderBy = null, ?int $limit = null, ?int $offset = null): array;
+}