fix(gateway): 分组隔离 — 禁止未分组账号被跨组调度

当 API Key 无分组时，调度仅从未分组账号池中选取。
修复 isAccountInGroup 在 groupID==nil 时的逻辑，
同时补全 scheduler_snapshot_service 和 gemini_compat_service
中的 SimpleMode 保护，确保分组隔离在所有调度路径生效。

新增 ListSchedulableUngroupedByPlatform/s 方法，
使用 Ent 的 Not(HasAccountGroups()) 谓词实现未分组账号隔离。
新增 17 个单元和端到端隔离测试，覆盖所有分支和边界条件。

backend/internal/repository/account_repo.go

@@ -829,6 +829,51 @@ func (r *accountRepository) ListSchedulableByPlatforms(ctx context.Context, plat
 	return r.accountsToService(ctx, accounts)
 }
 
+func (r *accountRepository) ListSchedulableUngroupedByPlatform(ctx context.Context, platform string) ([]service.Account, error) {
+	now := time.Now()
+	accounts, err := r.client.Account.Query().
+		Where(
+			dbaccount.PlatformEQ(platform),
+			dbaccount.StatusEQ(service.StatusActive),
+			dbaccount.SchedulableEQ(true),
+			dbaccount.Not(dbaccount.HasAccountGroups()),
+			tempUnschedulablePredicate(),
+			notExpiredPredicate(now),
+			dbaccount.Or(dbaccount.OverloadUntilIsNil(), dbaccount.OverloadUntilLTE(now)),
+			dbaccount.Or(dbaccount.RateLimitResetAtIsNil(), dbaccount.RateLimitResetAtLTE(now)),
+		).
+		Order(dbent.Asc(dbaccount.FieldPriority)).
+		All(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return r.accountsToService(ctx, accounts)
+}
+
+func (r *accountRepository) ListSchedulableUngroupedByPlatforms(ctx context.Context, platforms []string) ([]service.Account, error) {
+	if len(platforms) == 0 {
+		return nil, nil
+	}
+	now := time.Now()
+	accounts, err := r.client.Account.Query().
+		Where(
+			dbaccount.PlatformIn(platforms...),
+			dbaccount.StatusEQ(service.StatusActive),
+			dbaccount.SchedulableEQ(true),
+			dbaccount.Not(dbaccount.HasAccountGroups()),
+			tempUnschedulablePredicate(),
+			notExpiredPredicate(now),
+			dbaccount.Or(dbaccount.OverloadUntilIsNil(), dbaccount.OverloadUntilLTE(now)),
+			dbaccount.Or(dbaccount.RateLimitResetAtIsNil(), dbaccount.RateLimitResetAtLTE(now)),
+		).
+		Order(dbent.Asc(dbaccount.FieldPriority)).
+		All(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return r.accountsToService(ctx, accounts)
+}
+
 func (r *accountRepository) ListSchedulableByGroupIDAndPlatforms(ctx context.Context, groupID int64, platforms []string) ([]service.Account, error) {
 	if len(platforms) == 0 {
 		return nil, nil