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/store.go

@@ -107,6 +107,38 @@ func (s *Store) GetUsageRange(ctx context.Context, userID int64, from, to time.T
 	return out, rows.Err()
 }
 
+// HourBucket is one UTC epoch-hour's account usage (read from usage_hourly).
+type HourBucket struct {
+	Hour        int64 // unix epoch hour (UTC) = floor(unix_seconds / 3600)
+	BytesUp     uint64
+	BytesDown   uint64
+	MinutesUsed int
+}
+
+// HourlyRange returns usage_hourly rows for userID with hour in [fromHour, toHour]
+// (inclusive, epoch hours), oldest first. Empty hours are simply absent.
+func (s *Store) HourlyRange(ctx context.Context, userID, fromHour, toHour int64) ([]HourBucket, error) {
+	rows, err := s.db.QueryContext(ctx,
+		`SELECT hour, bytes_up, bytes_down, minutes_used
+		 FROM usage_hourly
+		 WHERE user_id = ? AND hour BETWEEN ? AND ?
+		 ORDER BY hour ASC`,
+		userID, fromHour, toHour)
+	if err != nil {
+		return nil, fmt.Errorf("store.HourlyRange: %w", err)
+	}
+	defer rows.Close()
+	var out []HourBucket
+	for rows.Next() {
+		var b HourBucket
+		if err := rows.Scan(&b.Hour, &b.BytesUp, &b.BytesDown, &b.MinutesUsed); err != nil {
+			return nil, fmt.Errorf("store.HourlyRange scan: %w", err)
+		}
+		out = append(out, b)
+	}
+	return out, rows.Err()
+}
+
 // DeviceUsageRow is one device's aggregated usage over a date window, joined to
 // the devices table for display metadata.  It is the per-device ("下分设备")
 // counterpart of DailyUsage's account rollup.