fix(service-desk): make carnet_id/member_id nullable for walk-in guests

The service desk ticket form allows issuing tickets to walk-in guests
without a carnet or membership, but the carnet_guest_entries table had
NOT NULL + FK constraints on both columns, causing a 500 error.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

database/migrations/Phase_100_011_make_carnet_guest_entries_nullable.php

+<?php
+declare(strict_types=1);
+
+return function (\App\Core\Database $db): void {
+    $db->raw("ALTER TABLE carnet_guest_entries DROP FOREIGN KEY fk_cge_carnet");
+    $db->raw("ALTER TABLE carnet_guest_entries DROP FOREIGN KEY fk_cge_member");
+
+    $db->raw("ALTER TABLE carnet_guest_entries MODIFY carnet_id BIGINT UNSIGNED NULL DEFAULT NULL");
+    $db->raw("ALTER TABLE carnet_guest_entries MODIFY member_id BIGINT UNSIGNED NULL DEFAULT NULL");
+
+    $db->raw("ALTER TABLE carnet_guest_entries ADD CONSTRAINT fk_cge_carnet FOREIGN KEY (carnet_id) REFERENCES carnets(id) ON DELETE SET NULL");
+    $db->raw("ALTER TABLE carnet_guest_entries ADD CONSTRAINT fk_cge_member FOREIGN KEY (member_id) REFERENCES members(id) ON DELETE SET NULL");
+};