fix(usage): 统计按客户端本地日聚合 —— 修「29号却显示28号数据」时区 bug

根因:服务端用量全按 UTC 预切天,UTC+8 用户在中国 0-8 点时 UTC 还是前一天,
「今天」就显示前一天数据。

改法(用户拍板:小时桶+查询带时区偏移):
- migration 000017 `usage_hourly`(UTC 纪元小时整数桶,稀疏存储,空小时不落行)。
- ReportUsage 同时累加 usage_hourly(按 windowEnd 的 UTC 小时);usage_daily 保留作配额/今日。
- `UsageCurve(days, offsetMin)`:读 UTC 小时桶,按客户端时区偏移**重新聚合成本地日**曲线。
- `/v1/usage?tz_offset=分钟`;客户端 `usage()` 带 `DateTime.now().timeZoneOffset.inMinutes`。
- 客户端早已「取曲线最后一点当今日」,故只改服务端聚合 + 带偏移即可。

测试:store 时区分桶证明（UTC 18:00 在 UTC vs UTC+8 落不同本地日,锁住 bug 修复）+
migration v17 + usage_hourly 表;集成测试改喂 hourly;全量 go/flutter test 绿。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/internal/usage/service.go

@@ -46,38 +46,59 @@ type UsagePoint struct {
 	MinutesUsed int    `json:"minutes_used"`
 }
 
-// UsageCurve returns the last `days` daily points for userID (UTC), with
-// missing days zero-filled, oldest first.  days is clamped to [1, 90].
-func (svc *Service) UsageCurve(ctx context.Context, userID int64, days int) ([]UsagePoint, *apierr.Error) {
+// UsageCurve returns the last `days` usage points for userID bucketed in the
+// CLIENT's timezone (offsetMin = client UTC offset in minutes, e.g. 480 for
+// UTC+8), missing days zero-filled, oldest first. days clamped to [1,90].
+//
+// It reads UTC hourly buckets (usage_hourly) and re-aggregates by local calendar
+// day, so "today" matches the user's local date regardless of server timezone
+// (fixes "29号却显示28号数据"). offsetMin out of ±14h falls back to 0 (UTC).
+func (svc *Service) UsageCurve(ctx context.Context, userID int64, days, offsetMin int) ([]UsagePoint, *apierr.Error) {
 	if days < 1 {
 		days = 7
 	}
 	if days > 90 {
 		days = 90
 	}
+	if offsetMin < -14*60 || offsetMin > 14*60 {
+		offsetMin = 0
+	}
+	loc := time.FixedZone("client", offsetMin*60)
 
-	today := utcToday()
-	from := today.AddDate(0, 0, -(days - 1))
+	now := nowFunc().UTC()
+	ln := now.In(loc)
+	localToday := time.Date(ln.Year(), ln.Month(), ln.Day(), 0, 0, 0, 0, loc)
+	fromLocal := localToday.AddDate(0, 0, -(days - 1))
 
-	rows, err := svc.store.GetUsageRange(ctx, userID, from, today)
+	fromHour := fromLocal.UTC().Unix() / 3600
+	toHour := now.Unix() / 3600
+
+	rows, err := svc.store.HourlyRange(ctx, userID, fromHour, toHour)
 	if err != nil {
 		return nil, apierr.ErrInternal
 	}
-	byDate := make(map[string]DailyUsage, len(rows))
-	for _, r := range rows {
-		byDate[r.Date.UTC().Format(dateLayout)] = r
+	type agg struct {
+		up, down uint64
+		min      int
+	}
+	byDay := make(map[string]*agg)
+	for _, b := range rows {
+		key := time.Unix(b.Hour*3600, 0).In(loc).Format(dateLayout)
+		a := byDay[key]
+		if a == nil {
+			a = &agg{}
+			byDay[key] = a
+		}
+		a.up += b.BytesUp
+		a.down += b.BytesDown
+		a.min += b.MinutesUsed
 	}
 
 	points := make([]UsagePoint, 0, days)
 	for i := 0; i < days; i++ {
-		d := from.AddDate(0, 0, i).Format(dateLayout)
-		if u, ok := byDate[d]; ok {
-			points = append(points, UsagePoint{
-				Date:        d,
-				BytesUp:     u.BytesUp,
-				BytesDown:   u.BytesDown,
-				MinutesUsed: u.MinutesUsed,
-			})
+		d := fromLocal.AddDate(0, 0, i).Format(dateLayout)
+		if a, ok := byDay[d]; ok {
+			points = append(points, UsagePoint{Date: d, BytesUp: a.up, BytesDown: a.down, MinutesUsed: a.min})
 		} else {
 			points = append(points, UsagePoint{Date: d})
 		}