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/internal/config/config.go

@@ -13,6 +13,7 @@ type Config struct {
 	ClassesServiceURL string
 	IamServiceURL     string
 	TeacherBffURL     string
+	CoreEduServiceURL string
 	OTLPEndpoint      string
 	LogLevel          string
 	DevMode           bool
@@ -27,6 +28,7 @@ func Load() *Config {
 		ClassesServiceURL: getEnv("CLASSES_SERVICE_URL", "http://localhost:3001"),
 		IamServiceURL:     getEnv("IAM_SERVICE_URL", "http://localhost:3002"),
 		TeacherBffURL:     getEnv("TEACHER_BFF_URL", "http://localhost:3003"),
+		CoreEduServiceURL: getEnv("CORE_EDU_SERVICE_URL", "http://localhost:3004"),
 		OTLPEndpoint:      getEnv("OTEL_EXPORTER_OTLP_ENDPOINT", "http://localhost:4318"),
 		LogLevel:          getEnv("LOG_LEVEL", "info"),
 		DevMode:           getEnvBool("DEV_MODE", false),