mcp: add image url support (#1)

* refactor: remove redundant validation in service and clean up code

- Remove validatePublishRequest function as gin binding already handles validation
- Remove unused errors import
- Simplify PublishContent method by relying on gin's built-in validation
- Add comprehensive image processing support with URL download capability

* 添加 MCP 说明文档，完善教程

go.mod

@@ -5,6 +5,7 @@ go 1.23.5
 require (
 	github.com/gin-gonic/gin v1.10.1
 	github.com/go-rod/rod v0.116.2
+	github.com/h2non/filetype v1.1.3
 	github.com/pkg/errors v0.9.1
 	github.com/sirupsen/logrus v1.9.3
 	github.com/stretchr/testify v1.10.0