fix: 부서 트리 API 테넌트 필터링 및 사원 연동 수정

- DepartmentUser: Model → Pivot 상속 변경 (fromRawAttributes 에러 수정)
- Department: BelongsToTenant 트레이트 추가 (테넌트 필터링)
- Department.employees(): tenant_user_profiles 기반 사원 조회 관계 추가
- DepartmentService.tree(): department_user 대신 employees 사용
- users 형태로 변환하여 프론트엔드 호환성 유지

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/Models/Tenants/Department.php

@@ -5,16 +5,18 @@
 use App\Models\Members\User;
 use App\Models\Permissions\PermissionOverride;
 use App\Models\Tenants\Pivots\DepartmentUser;
+use App\Traits\BelongsToTenant;
 use App\Traits\ModelTrait;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\HasMany;
 use Illuminate\Database\Eloquent\Relations\MorphMany;
 use Illuminate\Database\Eloquent\SoftDeletes;
 use Spatie\Permission\Traits\HasRoles;
 
 class Department extends Model
 {
-    use HasRoles, ModelTrait, SoftDeletes; // 부서도 권한/역할을 가짐
+    use BelongsToTenant, HasRoles, ModelTrait, SoftDeletes; // 부서도 권한/역할을 가짐
 
     protected $table = 'departments';
 
@@ -50,6 +52,7 @@ public function users()
     {
         return $this->belongsToMany(User::class, 'department_user')
             ->using(DepartmentUser::class)
+            ->wherePivotNull('deleted_at')
             ->withTimestamps()
             ->withPivot(['tenant_id', 'is_primary', 'joined_at', 'left_at']);
     }
@@ -65,4 +68,13 @@ public function departmentUsers()
     {
         return $this->hasMany(DepartmentUser::class, 'department_id');
     }
+
+    /** 부서 소속 사원 (tenant_user_profiles 기반) */
+    public function employees(): HasMany
+    {
+        // Department가 이미 BelongsToTenant로 필터링되므로
+        // department_id 연결만으로 테넌트 격리됨
+        return $this->hasMany(TenantUserProfile::class, 'department_id')
+            ->where('employee_status', 'active');
+    }
 }

app/Models/Tenants/Pivots/DepartmentUser.php

@@ -7,13 +7,13 @@
 use App\Models\Tenants\Department;
 use App\Traits\BelongsToTenant;
 use App\Traits\ModelTrait;
-use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\Pivot;
 use Illuminate\Database\Eloquent\SoftDeletes;
 
 /**
  * @mixin IdeHelperDepartmentUser
  */
-class DepartmentUser extends Model
+class DepartmentUser extends Pivot
 {
     use BelongsToTenant, ModelTrait, SoftDeletes;