docs(core): add KDoc documentation to controllers, services, and entities

Add descriptive KDoc comments to all REST controllers, service interfaces,
entity classes, and mappers to improve code readability and maintainability.
Include annotations for controller-level API documentation.

src/main/kotlin/com/msksbr/bookmgr/controller/AdminBookController.kt

@@ -2,6 +2,9 @@ package com.msksbr.bookmgr.controller
 
 import org.springframework.web.bind.annotation.RestController
 
+/*
+* 管理端图书接口
+*/
 @RestController
 class AdminBookController {
 }