feat: [org-chart] 조직도 관리 API 이관 (8개 엔드포인트)

- OrgChartController + OrgChartService 신규 생성
- FormRequest 5개 (Assign/Unassign/ReorderEmployees/ReorderDepartments/ToggleHide)
- Department 모델 options cast 추가
- Swagger 문서 (OrgChartApi.php) 생성
- hr.php 라우트 그룹 추가 (/v1/org-chart)

app/Http/Requests/OrgChart/ReorderEmployeesRequest.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace App\Http\Requests\OrgChart;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+class ReorderEmployeesRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    public function rules(): array
+    {
+        return [
+            'moves' => 'required|array',
+            'moves.*.employee_id' => 'required|integer',
+            'moves.*.department_id' => 'nullable|integer',
+        ];
+    }
+}