fix(token-cache): 修复异步刷新与请求线程的缓存竞态条件

- 新增 _token_version 版本号机制，防止过期 token 污染缓存
- TokenRefreshService 刷新成功后写入版本号并清除缓存
- TokenProvider 写入缓存前检查版本，过时则跳过
- ClearError 时同步清除 token 缓存

backend/internal/server/api_contract_test.go

@@ -193,20 +193,20 @@ func TestAPIContracts(t *testing.T) {
 				// 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,
+						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,
+						AssignedBy:      ptr(int64(999)),
+						AssignedAt:      deps.now,
+						Notes:           "admin-note",
+						CreatedAt:       deps.now,
+						UpdatedAt:       deps.now,
 					},
 				})
 			},