fix(quota): 平台限额表只保存已配置限额的记录，仪表盘按用量与限额展示平台

user_platform_quotas 只保存至少配置了一档限额的记录；不存在的行等价于不限额。

- 注册播种只为配置了限额的平台建行，一个都没有时不写入
- IncrementUsageWithReset 无行时不再 fail-open 建行
- BatchSnapshotUsage 改为只更新既有活跃行，不再插入无限额的新行
- 仓储 BulkInsertInitial/UpsertForUser 把三档全空的记录视为未配置；
  管理员全量替换接口同样剔除，清空限额即软删并放弃累计用量
- 迁移 238 删除三档全 NULL 的历史行
- 用户仪表盘平台卡片 = 有用量的平台 ∪ 配置了限额的平台，计数改为卡片数
- 管理员限额弹窗对未保存限额的平台禁用"重置窗口"

backend/internal/handler/admin/user_handler.go

@@ -790,15 +790,21 @@ func (h *UserHandler) UpdateUserPlatformQuotas(c *gin.Context) {
 		}
 	}
 
+	// 三档全空的输入不落库：user_platform_quotas 只保存至少配置了一档限额的记录，
+	// 不存在的行等价于不限额。未进入 records 的平台由 UpsertForUser 软删，审计里记为 removed。
 	records := make([]service.UserPlatformQuotaRecord, 0, len(req.Quotas))
 	for _, q := range req.Quotas {
-		records = append(records, service.UserPlatformQuotaRecord{
+		rec := service.UserPlatformQuotaRecord{
 			UserID:          userID,
 			Platform:        q.Platform,
 			DailyLimitUSD:   q.DailyLimitUSD,
 			WeeklyLimitUSD:  q.WeeklyLimitUSD,
 			MonthlyLimitUSD: q.MonthlyLimitUSD,
-		})
+		}
+		if !rec.HasAnyLimit() {
+			continue
+		}
+		records = append(records, rec)
 	}
 
 	ctx := c.Request.Context()
@@ -860,6 +866,7 @@ func (h *UserHandler) UpdateUserPlatformQuotas(c *gin.Context) {
 	slog.Info("admin.quota_updated",
 		"actor_admin_id", getAdminIDFromContext(c),
 		"target_user_id", userID,
+		"submitted_count", len(req.Quotas),
 		"platform_count", len(records),
 		"before_snapshot_available", beforeErr == nil,
 		"changes", changes)

backend/internal/handler/admin/user_platform_quota_admin_test.go

@@ -112,16 +112,61 @@ func TestUpdateUserPlatformQuotas_Success(t *testing.T) {
 	if len(repo.upsertCalls) != 1 {
 		t.Fatalf("UpsertForUser should be called once, got %d", len(repo.upsertCalls))
 	}
-	// upsert 记录数 = 请求体中给出的平台数（未给出的平台不落库）。
-	if repo.upsertCalls[0].userID != 42 || len(repo.upsertCalls[0].records) != 5 {
+	// upsert 记录数 = 请求体中至少配置了一档限额的平台数；三档全空的平台不落库，
+	// 由 UpsertForUser 的"不在列表里就软删"分支处理。
+	if repo.upsertCalls[0].userID != 42 || len(repo.upsertCalls[0].records) != 2 {
 		t.Errorf("unexpected upsert call: %+v", repo.upsertCalls[0])
 	}
+	for _, r := range repo.upsertCalls[0].records {
+		if r.Platform != "anthropic" && r.Platform != "openai" {
+			t.Errorf("platform %q has no configured limit and must not be upserted", r.Platform)
+		}
+	}
 	// 缓存失效：按全部允许平台统一失效（含 kimi/zhipu/deepseek）。
 	if len(cache.deleteCalls) != len(service.AllowedQuotaPlatforms) {
 		t.Errorf("expected %d cache delete calls, got %d: %+v", len(service.AllowedQuotaPlatforms), len(cache.deleteCalls), cache.deleteCalls)
 	}
 }
 
+// TestUpdateUserPlatformQuotas_AllUnlimitedClearsRows 锁定：全部平台三档全空等价于清空，
+// UpsertForUser 收到空列表（软删该用户所有活跃行），且 0 = 显式禁用仍算已配置。
+func TestUpdateUserPlatformQuotas_AllUnlimitedClearsRows(t *testing.T) {
+	repo := &upsertCapturingQuotaRepo{}
+	cache := &billingCacheStub{}
+	h := buildTestHandler(repo, cache)
+
+	body := `{"quotas":[
+		{"platform":"anthropic","daily_limit_usd":null,"weekly_limit_usd":null,"monthly_limit_usd":null},
+		{"platform":"openai"}
+	]}`
+	c, w := putReq(t, body)
+	h.UpdateUserPlatformQuotas(c)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", w.Code, w.Body.String())
+	}
+	if len(repo.upsertCalls) != 1 {
+		t.Fatalf("UpsertForUser should be called once, got %d", len(repo.upsertCalls))
+	}
+	if len(repo.upsertCalls[0].records) != 0 {
+		t.Errorf("all-unlimited input must upsert zero records, got %+v", repo.upsertCalls[0].records)
+	}
+
+	repo = &upsertCapturingQuotaRepo{}
+	h = buildTestHandler(repo, &billingCacheStub{})
+	c, w = putReq(t, `{"quotas":[{"platform":"gemini","daily_limit_usd":0}]}`)
+	h.UpdateUserPlatformQuotas(c)
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", w.Code, w.Body.String())
+	}
+	if len(repo.upsertCalls) != 1 || len(repo.upsertCalls[0].records) != 1 {
+		t.Fatalf("zero limit is a configured limit and must be upserted: %+v", repo.upsertCalls)
+	}
+	if r := repo.upsertCalls[0].records[0]; r.Platform != "gemini" || r.DailyLimitUSD == nil || *r.DailyLimitUSD != 0 {
+		t.Errorf("unexpected record: %+v", r)
+	}
+}
+
 func TestUpdateUserPlatformQuotas_RejectsDuplicatePlatform(t *testing.T) {
 	h := buildTestHandler(&upsertCapturingQuotaRepo{}, &billingCacheStub{})
 	body := `{"quotas":[