fix(subscriptions): 用户订阅不返回分配信息

- 用户侧 UserSubscription DTO 移除 assigned_by/assigned_at/notes/assigned_by_user 等管理员字段\n- 新增 AdminUserSubscription，并调整管理员订阅接口与批量分配结果使用\n- 增加 /api/v1/subscriptions 契约测试，确保用户侧响应不包含上述字段

backend/internal/server/api_contract_test.go

@@ -186,6 +186,56 @@ func TestAPIContracts(t *testing.T) {
 				]
 			}`,
 		},
+		{
+			name: "GET /api/v1/subscriptions",
+			setup: func(t *testing.T, deps *contractDeps) {
+				t.Helper()
+				// 普通用户订阅接口不应包含 assigned_* / notes 等管理员字段。
+				deps.userSubRepo.SetByUserID(1, []service.UserSubscription{
+					{
+						ID:           501,
+						UserID:       1,
+						GroupID:      10,
+						StartsAt:     deps.now,
+						ExpiresAt:    deps.now.Add(24 * time.Hour),
+						Status:       service.SubscriptionStatusActive,
+						DailyUsageUSD:   1.23,
+						WeeklyUsageUSD:  2.34,
+						MonthlyUsageUSD: 3.45,
+						AssignedBy:    ptr(int64(999)),
+						AssignedAt:    deps.now,
+						Notes:         "admin-note",
+						CreatedAt:     deps.now,
+						UpdatedAt:     deps.now,
+					},
+				})
+			},
+			method:     http.MethodGet,
+			path:       "/api/v1/subscriptions",
+			wantStatus: http.StatusOK,
+			wantJSON: `{
+				"code": 0,
+				"message": "success",
+				"data": [
+					{
+						"id": 501,
+						"user_id": 1,
+						"group_id": 10,
+						"starts_at": "2025-01-02T03:04:05Z",
+						"expires_at": "2025-01-03T03:04:05Z",
+						"status": "active",
+						"daily_window_start": null,
+						"weekly_window_start": null,
+						"monthly_window_start": null,
+						"daily_usage_usd": 1.23,
+						"weekly_usage_usd": 2.34,
+						"monthly_usage_usd": 3.45,
+						"created_at": "2025-01-02T03:04:05Z",
+						"updated_at": "2025-01-02T03:04:05Z"
+					}
+				]
+			}`,
+		},
 		{
 			name: "GET /api/v1/usage/stats",
 			setup: func(t *testing.T, deps *contractDeps) {
@@ -490,6 +540,9 @@ func newContractDeps(t *testing.T) *contractDeps {
 	usageRepo := newStubUsageLogRepo()
 	usageService := service.NewUsageService(usageRepo, userRepo, nil, nil)
 
+	subscriptionService := service.NewSubscriptionService(groupRepo, userSubRepo, nil)
+	subscriptionHandler := handler.NewSubscriptionHandler(subscriptionService)
+
 	settingRepo := newStubSettingRepo()
 	settingService := service.NewSettingService(settingRepo, cfg)
 
@@ -536,6 +589,10 @@ func newContractDeps(t *testing.T) *contractDeps {
 	v1Usage.GET("/usage", usageHandler.List)
 	v1Usage.GET("/usage/stats", usageHandler.Stats)
 
+	v1Subs := v1.Group("")
+	v1Subs.Use(jwtAuth)
+	v1Subs.GET("/subscriptions", subscriptionHandler.List)
+
 	v1Admin := v1.Group("/admin")
 	v1Admin.Use(adminAuth)
 	v1Admin.GET("/settings", adminSettingHandler.GetSettings)