refactor(backend): 添加 service 缓存端口

backend/internal/service/ports/update_cache.go

@@ -0,0 +1,12 @@
+package ports
+
+import (
+	"context"
+	"time"
+)
+
+// UpdateCache defines cache operations for update service
+type UpdateCache interface {
+	GetUpdateInfo(ctx context.Context) (string, error)
+	SetUpdateInfo(ctx context.Context, data string, ttl time.Duration) error
+}