Fix compatibilité MySQL : NULLS LAST et order withCount/select

- DbCompat::nullsLast() : syntaxe portable pgsql/mysql pour ORDER BY NULLS LAST
- RechercheController, ExportController : remplace 'nom ASC NULLS LAST' (pgsql only)
- CarteController : select() avant withCount() pour ne pas effacer le COUNT subquery

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Http/Controllers/CarteController.php

@@ -21,8 +21,8 @@ class CarteController extends Controller
             ->whereHas('sources.releves')
             ->with([
                 'sources' => function ($q) {
-                    $q->withCount('releves')
-                      ->select('id', 'nom', 'lieu_id', 'status', 'annee_debut', 'annee_fin')
+                    $q->select('id', 'nom', 'lieu_id', 'status', 'annee_debut', 'annee_fin')
+                      ->withCount('releves')
                       ->orderBy('nom');
                 },
             ])