feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/internal/handler/payment_webhook_handler_test.go

@@ -0,0 +1,99 @@
+//go:build unit
+
+package handler
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestWriteSuccessResponse(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	tests := []struct {
+		name            string
+		providerKey     string
+		wantCode        int
+		wantContentType string
+		wantBody        string
+		checkJSON       bool
+		wantJSONCode    string
+		wantJSONMessage string
+	}{
+		{
+			name:            "wxpay returns JSON with code SUCCESS",
+			providerKey:     "wxpay",
+			wantCode:        http.StatusOK,
+			wantContentType: "application/json",
+			checkJSON:       true,
+			wantJSONCode:    "SUCCESS",
+			wantJSONMessage: "成功",
+		},
+		{
+			name:            "stripe returns empty 200",
+			providerKey:     "stripe",
+			wantCode:        http.StatusOK,
+			wantContentType: "text/plain",
+			wantBody:        "",
+		},
+		{
+			name:            "easypay returns plain text success",
+			providerKey:     "easypay",
+			wantCode:        http.StatusOK,
+			wantContentType: "text/plain",
+			wantBody:        "success",
+		},
+		{
+			name:            "alipay returns plain text success",
+			providerKey:     "alipay",
+			wantCode:        http.StatusOK,
+			wantContentType: "text/plain",
+			wantBody:        "success",
+		},
+		{
+			name:            "unknown provider returns plain text success",
+			providerKey:     "unknown_provider",
+			wantCode:        http.StatusOK,
+			wantContentType: "text/plain",
+			wantBody:        "success",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			w := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(w)
+
+			writeSuccessResponse(c, tt.providerKey)
+
+			assert.Equal(t, tt.wantCode, w.Code)
+			assert.Contains(t, w.Header().Get("Content-Type"), tt.wantContentType)
+
+			if tt.checkJSON {
+				var resp wxpaySuccessResponse
+				err := json.Unmarshal(w.Body.Bytes(), &resp)
+				require.NoError(t, err, "response body should be valid JSON")
+				assert.Equal(t, tt.wantJSONCode, resp.Code)
+				assert.Equal(t, tt.wantJSONMessage, resp.Message)
+			} else {
+				assert.Equal(t, tt.wantBody, w.Body.String())
+			}
+		})
+	}
+}
+
+func TestWebhookConstants(t *testing.T) {
+	t.Run("maxWebhookBodySize is 1MB", func(t *testing.T) {
+		assert.Equal(t, int64(1<<20), int64(maxWebhookBodySize))
+	})
+
+	t.Run("webhookLogTruncateLen is 200", func(t *testing.T) {
+		assert.Equal(t, 200, webhookLogTruncateLen)
+	})
+}