fix: [hr] 사원 등록 시 user_tenants pivot 연동 추가

- User 생성 후 tenants()->attach() 호출 (멀티테넌트 필수)
- user_id, must_change_password, created_by 필드 추가
- 기존 UserService 패턴과 동일하게 맞춤

app/Services/HR/EmployeeService.php

@@ -93,13 +93,25 @@ public function createEmployee(array $data): Employee
         return DB::transaction(function () use ($data, $tenantId) {
             // User 생성
             $user = User::create([
+                'user_id' => $data['email'] ?? $data['name'],
                 'name' => $data['name'],
                 'email' => $data['email'] ?? null,
                 'phone' => $data['phone'] ?? null,
                 'password' => bcrypt($data['password'] ?? 'sam1234!'),
                 'is_active' => true,
+                'must_change_password' => true,
+                'created_by' => auth()->id(),
             ]);
 
+            // user_tenants pivot 연동 (멀티테넌트 필수)
+            if ($tenantId) {
+                $user->tenants()->attach($tenantId, [
+                    'is_active' => true,
+                    'is_default' => true,
+                    'joined_at' => now(),
+                ]);
+            }
+
             // json_extra 구성
             $jsonExtra = [];
             if (! empty($data['employee_code'])) {