feat(mcp): integrate MCP Server into backend service as built-in endpoint

- Add MCP Controller and register /mcp route
- MCP Server now uses OpenAPI Token for authentication
- Update frontend settings to show built-in MCP endpoint config
- Update docs to reflect integrated MCP endpoint

internal/controllers/mcp_controller.go

@@ -0,0 +1,19 @@
+package controllers
+
+import (
+	"github.com/engigu/taskpool/internal/mcp"
+	"github.com/gin-gonic/gin"
+)
+
+type MCPController struct{}
+
+func NewMCPController() *MCPController {
+	return &MCPController{}
+}
+
+// HandleMCP 处理 MCP 请求（StreamableHTTP）
+func (c *MCPController) HandleMCP(ctx *gin.Context) {
+	// 获取 MCP HTTP Handler 并处理请求
+	handler := mcp.GetHTTPHandler()
+	handler.ServeHTTP(ctx.Writer, ctx.Request)
+}