fix(admin): restore currency and timestamps in admin plan list response

The composite-groups PR (#3581) replaced the raw ent SubscriptionPlan
response of GET /admin/payment/plans with a projection struct but
dropped the currency field added by #4323. PlanEditDialog then read an
undefined currency, sent an empty string on save, and silently wiped
the stored plan currency. Restore currency plus created_at/updated_at
so the projection preserves the full original response shape.

backend/internal/handler/admin/payment_handler.go

@@ -288,25 +288,28 @@ func (h *PaymentHandler) ListPlans(c *gin.Context) {
 }
 
 type AdminSubscriptionPlanResult struct {
-	ID              int64    `json:"id"`
-	GroupID         int64    `json:"group_id"`
-	GroupPlatform   string   `json:"group_platform,omitempty"`
-	GroupName       string   `json:"group_name,omitempty"`
-	RateMultiplier  float64  `json:"rate_multiplier,omitempty"`
-	DailyLimitUSD   *float64 `json:"daily_limit_usd,omitempty"`
-	WeeklyLimitUSD  *float64 `json:"weekly_limit_usd,omitempty"`
-	MonthlyLimitUSD *float64 `json:"monthly_limit_usd,omitempty"`
-	ModelScopes     []string `json:"supported_model_scopes,omitempty"`
-	Name            string   `json:"name"`
-	Description     string   `json:"description"`
-	Price           float64  `json:"price"`
-	OriginalPrice   *float64 `json:"original_price,omitempty"`
-	ValidityDays    int      `json:"validity_days"`
-	ValidityUnit    string   `json:"validity_unit"`
-	Features        string   `json:"features"`
-	ProductName     string   `json:"product_name"`
-	ForSale         bool     `json:"for_sale"`
-	SortOrder       int      `json:"sort_order"`
+	ID              int64     `json:"id"`
+	GroupID         int64     `json:"group_id"`
+	GroupPlatform   string    `json:"group_platform,omitempty"`
+	GroupName       string    `json:"group_name,omitempty"`
+	RateMultiplier  float64   `json:"rate_multiplier,omitempty"`
+	DailyLimitUSD   *float64  `json:"daily_limit_usd,omitempty"`
+	WeeklyLimitUSD  *float64  `json:"weekly_limit_usd,omitempty"`
+	MonthlyLimitUSD *float64  `json:"monthly_limit_usd,omitempty"`
+	ModelScopes     []string  `json:"supported_model_scopes,omitempty"`
+	Name            string    `json:"name"`
+	Description     string    `json:"description"`
+	Price           float64   `json:"price"`
+	OriginalPrice   *float64  `json:"original_price,omitempty"`
+	Currency        string    `json:"currency,omitempty"`
+	ValidityDays    int       `json:"validity_days"`
+	ValidityUnit    string    `json:"validity_unit"`
+	Features        string    `json:"features"`
+	ProductName     string    `json:"product_name"`
+	ForSale         bool      `json:"for_sale"`
+	SortOrder       int       `json:"sort_order"`
+	CreatedAt       time.Time `json:"created_at,omitempty"`
+	UpdatedAt       time.Time `json:"updated_at,omitempty"`
 }
 
 func adminSubscriptionPlansForResponse(plans []*dbent.SubscriptionPlan, groupInfo map[int64]service.PlanGroupInfo) []AdminSubscriptionPlanResult {
@@ -330,12 +333,15 @@ func adminSubscriptionPlansForResponse(plans []*dbent.SubscriptionPlan, groupInf
 			Description:     p.Description,
 			Price:           p.Price,
 			OriginalPrice:   p.OriginalPrice,
+			Currency:        p.Currency,
 			ValidityDays:    p.ValidityDays,
 			ValidityUnit:    p.ValidityUnit,
 			Features:        p.Features,
 			ProductName:     p.ProductName,
 			ForSale:         p.ForSale,
 			SortOrder:       p.SortOrder,
+			CreatedAt:       p.CreatedAt,
+			UpdatedAt:       p.UpdatedAt,
 		})
 	}
 	return result

backend/internal/handler/admin/payment_handler_test.go

@@ -50,6 +50,7 @@ func TestSanitizeAdminPaymentOrderForResponseAddsCurrency(t *testing.T) {
 
 func TestAdminSubscriptionPlansForResponseIncludesCompositeGroupInfo(t *testing.T) {
 	weekly := 25.0
+	now := time.Now()
 	plans := []*dbent.SubscriptionPlan{
 		{
 			ID:           11,
@@ -57,12 +58,15 @@ func TestAdminSubscriptionPlansForResponseIncludesCompositeGroupInfo(t *testing.
 			Name:         "All models",
 			Description:  "Composite access",
 			Price:        19.99,
+			Currency:     "CNY",
 			ValidityDays: 30,
 			ValidityUnit: "days",
 			Features:     "OpenAI\nClaude\nGemini\nGrok",
 			ProductName:  "Sub2API",
 			ForSale:      true,
 			SortOrder:    1,
+			CreatedAt:    now,
+			UpdatedAt:    now,
 		},
 	}
 	groupInfo := map[int64]service.PlanGroupInfo{
@@ -92,4 +96,12 @@ func TestAdminSubscriptionPlansForResponseIncludesCompositeGroupInfo(t *testing.
 	if strings.Join(got[0].ModelScopes, ",") != "openai,claude,gemini,grok" {
 		t.Fatalf("expected model scopes to be preserved, got %#v", got[0].ModelScopes)
 	}
+	// 投影必须保留 ent 原始响应的全部套餐字段：currency 丢失曾导致编辑保存时
+	// 静默清空套餐货币（PlanEditDialog 回传空串 → SetCurrency("")）。
+	if got[0].Currency != "CNY" {
+		t.Fatalf("expected currency to be preserved, got %q", got[0].Currency)
+	}
+	if !got[0].CreatedAt.Equal(now) || !got[0].UpdatedAt.Equal(now) {
+		t.Fatalf("expected created_at/updated_at to be preserved, got %v / %v", got[0].CreatedAt, got[0].UpdatedAt)
+	}
 }