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.

internal/api/operation_log_handler.go

@@ -48,8 +48,8 @@ func toOperationLogView(l model.OperationLog) OperationLogView {
 // @Param        page    query     int     false  "page (1-based)"
 // @Param        limit   query     int     false  "limit (default 50, max 200)"
 // @Param        search  query     string  false  "search by actor/method/path"
-// @Success      200  {array}   OperationLogView
-// @Failure      500  {object}  gin.H
+// @Success      200  {object}  ResponseEnvelope{data=[]OperationLogView}
+// @Failure      500  {object}  ResponseEnvelope{data=gin.H}
 // @Router       /admin/operation-logs [get]
 func (h *AdminHandler) ListOperationLogs(c *gin.Context) {
 	var rows []model.OperationLog