✨ feat(department): add parent department ID field with default root value

- add parentId column to department entity with default 0 for root department
- update validation schema: change parentId from positive to nonnegative and set default 0
- update create/update DTOs to reflect parentId default value and description
- add comment for parentId field indicating 0 represents top-level department

src/server/modules/departments/department.entity.ts

@@ -30,6 +30,9 @@ export class Department {
   @Column({ name: 'is_active', type: 'tinyint', default: 1, comment: '是否启用(0:禁用,1:启用)' })
   isActive!: number;
 
+  @Column({ name: 'parent_id', type: 'int', unsigned: true, nullable: true, default: 0, comment: '父部门ID，0表示顶级部门' })
+  parentId?: number;
+
   @ManyToOne(() => Department, dept => dept.children)
   @JoinColumn({ name: 'parent_id' })
   parent?: Department;
@@ -87,7 +90,7 @@ export const DepartmentSchema = z.object({
   sortOrder: z.number().int().default(0).openapi({ description: '排序', example: 0 }),
   isActive: z.number().int().min(0).max(1).default(1).openapi({ description: '是否启用', example: 1 }),
   managerId: z.number().int().positive().nullable().openapi({ description: '部门负责人ID', example: 1 }),
-  parentId: z.number().int().positive().nullable().openapi({ description: '父部门ID', example: 1 }),
+  parentId: z.number().int().nonnegative().default(0).openapi({ description: '父部门ID，0表示顶级部门', example: 0 }),
   createdAt: z.date().openapi({ description: '创建时间' }),
   updatedAt: z.date().openapi({ description: '更新时间' })
 });
@@ -99,7 +102,7 @@ export const CreateDepartmentDto = z.object({
   sortOrder: z.number().int().optional().default(0).openapi({ description: '排序', example: 0 }),
   isActive: z.number().int().min(0).max(1).optional().default(1).openapi({ description: '是否启用', example: 1 }),
   managerId: z.number().int().positive().optional().openapi({ description: '部门负责人ID', example: 1 }),
-  parentId: z.number().int().positive().optional().openapi({ description: '父部门ID', example: 1 })
+  parentId: z.number().int().nonnegative().optional().default(0).openapi({ description: '父部门ID，0表示顶级部门', example: 0 })
 });
 
 export const UpdateDepartmentDto = z.object({
@@ -109,7 +112,7 @@ export const UpdateDepartmentDto = z.object({
   sortOrder: z.number().int().optional().openapi({ description: '排序', example: 0 }),
   isActive: z.number().int().min(0).max(1).optional().openapi({ description: '是否启用', example: 1 }),
   managerId: z.number().int().positive().optional().openapi({ description: '部门负责人ID', example: 1 }),
-  parentId: z.number().int().positive().optional().openapi({ description: '父部门ID', example: 1 })
+  parentId: z.number().int().nonnegative().optional().openapi({ description: '父部门ID，0表示顶级部门', example: 0 })
 });
 
 export const UserDepartmentSchema = z.object({