feat(api-gateway): 添加 core-edu 服务代理路由

main.go 新增 exams/homework/grades 三组路由(无尾斜杠+通配符)
config.go 新增 CoreEduServiceURL 字段默认 localhost:3004
删除死代码 internal/routing/routing.go(从未被 main 引用)

验证: GET /exams/class/:id 200, POST /exams 201, 链路全通

services/api-gateway/main.go

@@ -79,6 +79,20 @@ func main() {
 		bffHandler := proxy.ProxyHandler(bffProxy)
 		api.Any("/teacher", bffHandler)
 		api.Any("/teacher/*path", bffHandler)
+
+		// core-edu 服务路由（考试/作业/成绩）
+		// 注：同时注册无尾斜杠与通配符两条路由，与 classes/iam/teacher 一致。
+		coreEduProxy, err := proxy.NewProxy(cfg.CoreEduServiceURL)
+		if err != nil {
+			log.Fatalf("failed to create core-edu proxy: %v", err)
+		}
+		coreEduHandler := proxy.ProxyHandler(coreEduProxy)
+		api.Any("/exams", coreEduHandler)
+		api.Any("/exams/*path", coreEduHandler)
+		api.Any("/homework", coreEduHandler)
+		api.Any("/homework/*path", coreEduHandler)
+		api.Any("/grades", coreEduHandler)
+		api.Any("/grades/*path", coreEduHandler)
 	}
 
 	srv := &http.Server{