fix: quota display shows stale cumulative usage after daily/weekly reset

The quota reset mechanism is lazy — quota_daily_used/quota_weekly_used
in the database are only reset on the next IncrementQuotaUsed call.
The scheduling layer (IsQuotaExceeded) correctly checks period expiry
before enforcing limits, so the account remains usable. However, the
API response mapper reads the raw DB value without checking expiry,
causing the frontend to display cumulative usage (e.g. 110%) even
after the reset period has passed.

Add IsDailyQuotaPeriodExpired/IsWeeklyQuotaPeriodExpired methods and
use them in the mapper to return used=0 when the period has expired.

backend/internal/handler/dto/mappers.go

@@ -276,11 +276,17 @@ func AccountFromServiceShallow(a *service.Account) *Account {
 		if limit := a.GetQuotaDailyLimit(); limit > 0 {
 			out.QuotaDailyLimit = &limit
 			used := a.GetQuotaDailyUsed()
+			if a.IsDailyQuotaPeriodExpired() {
+				used = 0
+			}
 			out.QuotaDailyUsed = &used
 		}
 		if limit := a.GetQuotaWeeklyLimit(); limit > 0 {
 			out.QuotaWeeklyLimit = &limit
 			used := a.GetQuotaWeeklyUsed()
+			if a.IsWeeklyQuotaPeriodExpired() {
+				used = 0
+			}
 			out.QuotaWeeklyUsed = &used
 		}
 		// 固定时间重置配置