feat(redeem): support subscription type in create-and-redeem API

Add group_id and validity_days fields to CreateAndRedeemCodeRequest,
enabling subscription-type redemption codes to be created and redeemed
in a single API call.

- Type defaults to "balance" when omitted for backward compatibility
- Subscription type requires group_id (non-nil) and validity_days (>0)
- Existing balance/concurrency callers are unaffected

backend/internal/handler/admin/redeem_handler_test.go

@@ -0,0 +1,135 @@
+package admin
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// newCreateAndRedeemHandler creates a RedeemHandler with a non-nil (but minimal)
+// RedeemService so that CreateAndRedeem's nil guard passes and we can test the
+// parameter-validation layer that runs before any service call.
+func newCreateAndRedeemHandler() *RedeemHandler {
+	return &RedeemHandler{
+		adminService:  newStubAdminService(),
+		redeemService: &service.RedeemService{}, // non-nil to pass nil guard
+	}
+}
+
+// postCreateAndRedeemValidation calls CreateAndRedeem and returns the response
+// status code. For cases that pass validation and proceed into the service layer,
+// a panic may occur (because RedeemService internals are nil); this is expected
+// and treated as "validation passed" (returns 0 to indicate panic).
+func postCreateAndRedeemValidation(t *testing.T, handler *RedeemHandler, body any) (code int) {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+
+	jsonBytes, err := json.Marshal(body)
+	require.NoError(t, err)
+	c.Request, _ = http.NewRequest(http.MethodPost, "/api/v1/admin/redeem-codes/create-and-redeem", bytes.NewReader(jsonBytes))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	defer func() {
+		if r := recover(); r != nil {
+			// Panic means we passed validation and entered service layer (expected for minimal stub).
+			code = 0
+		}
+	}()
+	handler.CreateAndRedeem(c)
+	return w.Code
+}
+
+func TestCreateAndRedeem_TypeDefaultsToBalance(t *testing.T) {
+	// 不传 type 字段时应默认 balance，不触发 subscription 校验。
+	// 验证通过后进入 service 层会 panic（返回 0），说明默认值生效。
+	h := newCreateAndRedeemHandler()
+	code := postCreateAndRedeemValidation(t, h, map[string]any{
+		"code":    "test-balance-default",
+		"value":   10.0,
+		"user_id": 1,
+	})
+
+	assert.NotEqual(t, http.StatusBadRequest, code,
+		"omitting type should default to balance and pass validation")
+}
+
+func TestCreateAndRedeem_SubscriptionRequiresGroupID(t *testing.T) {
+	h := newCreateAndRedeemHandler()
+	code := postCreateAndRedeemValidation(t, h, map[string]any{
+		"code":          "test-sub-no-group",
+		"type":          "subscription",
+		"value":         29.9,
+		"user_id":       1,
+		"validity_days": 30,
+		// group_id 缺失
+	})
+
+	assert.Equal(t, http.StatusBadRequest, code)
+}
+
+func TestCreateAndRedeem_SubscriptionRequiresPositiveValidityDays(t *testing.T) {
+	groupID := int64(5)
+	h := newCreateAndRedeemHandler()
+
+	cases := []struct {
+		name         string
+		validityDays int
+	}{
+		{"zero", 0},
+		{"negative", -1},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			code := postCreateAndRedeemValidation(t, h, map[string]any{
+				"code":          "test-sub-bad-days-" + tc.name,
+				"type":          "subscription",
+				"value":         29.9,
+				"user_id":       1,
+				"group_id":      groupID,
+				"validity_days": tc.validityDays,
+			})
+
+			assert.Equal(t, http.StatusBadRequest, code)
+		})
+	}
+}
+
+func TestCreateAndRedeem_SubscriptionValidParamsPassValidation(t *testing.T) {
+	groupID := int64(5)
+	h := newCreateAndRedeemHandler()
+	code := postCreateAndRedeemValidation(t, h, map[string]any{
+		"code":          "test-sub-valid",
+		"type":          "subscription",
+		"value":         29.9,
+		"user_id":       1,
+		"group_id":      groupID,
+		"validity_days": 31,
+	})
+
+	assert.NotEqual(t, http.StatusBadRequest, code,
+		"valid subscription params should pass validation")
+}
+
+func TestCreateAndRedeem_BalanceIgnoresSubscriptionFields(t *testing.T) {
+	h := newCreateAndRedeemHandler()
+	// balance 类型不传 group_id 和 validity_days，不应报 400
+	code := postCreateAndRedeemValidation(t, h, map[string]any{
+		"code":    "test-balance-no-extras",
+		"type":    "balance",
+		"value":   50.0,
+		"user_id": 1,
+	})
+
+	assert.NotEqual(t, http.StatusBadRequest, code,
+		"balance type should not require group_id or validity_days")
+}