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

.claude/settings.local.json

@@ -6,7 +6,10 @@
       "Bash(go mod:*)",
       "Bash(go build:*)",
       "Bash(go get:*)",
-      "Bash(go list:*)"
+      "Bash(go list:*)",
+      "Bash(go test:*)",
+      "Bash(rmdir:*)",
+      "Bash(rm:*)"
     ],
     "deny": []
   }