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/usage_integration_test.go

@@ -438,11 +438,14 @@ func TestIntUsageCurveZeroFill(t *testing.T) {
 
 	uid := createUser(t, db, "dp-curve")
 	today := time.Now().UTC()
-	// Only two of the last 7 days have data.
-	store.AggregateUsage(context.Background(), uid, today, 1000, 2000, 5)
-	store.AggregateUsage(context.Background(), uid, today.AddDate(0, 0, -3), 50, 60, 2)
+	epochHour := func(tm time.Time) int64 { return tm.UTC().Unix() / 3600 }
+	// Only two of the last 7 days have data (curve reads hourly buckets now).
+	db.Exec(`INSERT INTO usage_hourly (user_id, hour, bytes_up, bytes_down, minutes_used) VALUES (?,?,?,?,?)`,
+		uid, epochHour(today), 1000, 2000, 5)
+	db.Exec(`INSERT INTO usage_hourly (user_id, hour, bytes_up, bytes_down, minutes_used) VALUES (?,?,?,?,?)`,
+		uid, epochHour(today.AddDate(0, 0, -3)), 50, 60, 2)
 
-	points, apiErr := svc.UsageCurve(context.Background(), uid, 7)
+	points, apiErr := svc.UsageCurve(context.Background(), uid, 7, 0)
 	if apiErr != nil {
 		t.Fatalf("curve: %v", apiErr)
 	}
@@ -479,10 +482,11 @@ func TestIntUsageCurveOnlyCurrentUser(t *testing.T) {
 
 	uA := createUser(t, db, "dp-A")
 	uB := createUser(t, db, "dp-B")
-	store.AggregateUsage(context.Background(), uA, time.Now().UTC(), 111, 0, 1)
-	store.AggregateUsage(context.Background(), uB, time.Now().UTC(), 999, 0, 9)
+	h := time.Now().UTC().Unix() / 3600
+	db.Exec(`INSERT INTO usage_hourly (user_id, hour, bytes_up, bytes_down, minutes_used) VALUES (?,?,?,?,?)`, uA, h, 111, 0, 1)
+	db.Exec(`INSERT INTO usage_hourly (user_id, hour, bytes_up, bytes_down, minutes_used) VALUES (?,?,?,?,?)`, uB, h, 999, 0, 9)
 
-	points, _ := svc.UsageCurve(context.Background(), uA, 1)
+	points, _ := svc.UsageCurve(context.Background(), uA, 1, 0)
 	if len(points) != 1 || points[0].BytesUp != 111 {
 		t.Errorf("user A curve leaked other user data: %+v", points)
 	}