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/namespace_handler_test.go

@@ -2,13 +2,13 @@ package api
 
 import (
 	"bytes"
-	"encoding/json"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"testing"
 
 	"github.com/alicebob/miniredis/v2"
+	"github.com/ez-api/ez-api/internal/middleware"
 	"github.com/ez-api/ez-api/internal/model"
 	"github.com/ez-api/ez-api/internal/service"
 	"github.com/gin-gonic/gin"
@@ -64,6 +64,7 @@ func TestNamespaceCRUD_DeleteCleansBindings(t *testing.T) {
 	}
 
 	r := gin.New()
+	r.Use(middleware.ResponseEnvelope())
 	r.POST("/admin/namespaces", h.CreateNamespace)
 	r.DELETE("/admin/namespaces/:id", h.DeleteNamespace)
 
@@ -77,9 +78,7 @@ func TestNamespaceCRUD_DeleteCleansBindings(t *testing.T) {
 		t.Fatalf("expected 201, got %d body=%s", rr.Code, rr.Body.String())
 	}
 	var created model.Namespace
-	if err := json.Unmarshal(rr.Body.Bytes(), &created); err != nil {
-		t.Fatalf("unmarshal: %v", err)
-	}
+	decodeEnvelope(t, rr, &created)
 
 	delReq := httptest.NewRequest(http.MethodDelete, fmt.Sprintf("/admin/namespaces/%d", created.ID), nil)
 	delRec := httptest.NewRecorder()