Fix: Persist manager update in department to database

This commit resolves the issue where changes to the department's manager were not being persisted to the database. Adjustments were made to ensure that the Department entity is properly managed within the transactional scope of the changeManager method.

src/main/kotlin/com/tistory/shanepark/dutypark/department/service/DepartmentService.kt

@@ -17,6 +17,7 @@ import org.springframework.transaction.annotation.Transactional
 class DepartmentService(
     private val repository: DepartmentRepository,
     private val dutyRepository: DutyRepository,
+    private val departmentRepository: DepartmentRepository
 ) {
 
     @Transactional(readOnly = true)
@@ -87,7 +88,7 @@ class DepartmentService(
 
     fun changeManager(department: Department, member: Member?) {
         department.changeManager(member)
+        departmentRepository.save(department)
     }
 
-
 }