refactor：重构安全模块架构，将security模块迁移至core层

- 将src/business/security模块迁移至src/core/security_core
- 更新模块导入路径和依赖关系
- 统一安全相关组件的命名规范（content_type.middleware.ts）
- 清理过时的配置文件和文档
- 更新架构文档以反映新的模块结构

此次重构符合业务功能模块化架构设计原则，将技术基础设施
服务统一放置在core层，提高代码组织的清晰度和可维护性。

src/app.module.ts

@@ -12,9 +12,9 @@ import { ZulipModule } from './business/zulip/zulip.module';
 import { RedisModule } from './core/redis/redis.module';
 import { AdminModule } from './business/admin/admin.module';
 import { UserMgmtModule } from './business/user-mgmt/user-mgmt.module';
-import { SecurityModule } from './business/security/security.module';
-import { MaintenanceMiddleware } from './business/security/middleware/maintenance.middleware';
-import { ContentTypeMiddleware } from './business/security/middleware/content-type.middleware';
+import { SecurityCoreModule } from './core/security_core/security_core.module';
+import { MaintenanceMiddleware } from './core/security_core/middleware/maintenance.middleware';
+import { ContentTypeMiddleware } from './core/security_core/middleware/content_type.middleware';
 
 /**
  * 检查数据库配置是否完整 by angjustinl 2025-12-17
@@ -71,7 +71,7 @@ function isDatabaseConfigured(): boolean {
     ZulipModule,
     UserMgmtModule,
     AdminModule,
-    SecurityModule,
+    SecurityCoreModule,
   ],
   controllers: [AppController],
   providers: [