feat(api): wrap JSON responses in envelope

Add response envelope middleware to standardize JSON responses as
`{code,data,message}` with consistent business codes across endpoints.
Update Swagger annotations and tests to reflect the new response shape.

BREAKING CHANGE: API responses are now wrapped in a response envelope; clients must read payloads from `data` and handle `code`/`message` fields.

cmd/server/main.go

@@ -292,6 +292,8 @@ func main() {
 		c.Next()
 	})
 
+	r.Use(middleware.ResponseEnvelope())
+
 	// 动态设置 Swagger Host
 	if cfg.Server.SwaggerHost != "" {
 		docs.SwaggerInfo.Host = cfg.Server.SwaggerHost