fix: batch 1 audit fixes — quota SQL fixed mode, public recharge URL, WebSearch bool fallback, UpdatePlan validation

H1: incrementUsageBillingAccountQuota now uses shared dailyExpiredExpr/weeklyExpiredExpr
    constants (supporting fixed reset mode) instead of hardcoded '24 hours'/'168 hours'
H4: public settings endpoint now maps balance_low_notify_recharge_url
H6: GetWebSearchEmulationMode tolerates legacy bool values (true→enabled)
H7: UpdatePlan validates non-nil patch fields (rejects negative price, empty name, etc.)
H8: UsageTable accountBilled() helper with total_cost ?? 0 null guard
H9: AdminUsageLog TS type adds channel_id + billing_tier
M2: account.go "fixed" literals replaced with thresholdTypeFixed constant
M13: SystemSettings TS type adds web_search_emulation_enabled
UI: QuotaLimitCard title labels now use flex-1 to align with flex-1 input boxes

backend/internal/handler/setting_handler.go

@@ -64,5 +64,6 @@ func (h *SettingHandler) GetPublicSettings(c *gin.Context) {
 		BalanceLowNotifyEnabled:          settings.BalanceLowNotifyEnabled,
 		AccountQuotaNotifyEnabled:        settings.AccountQuotaNotifyEnabled,
 		BalanceLowNotifyThreshold:        settings.BalanceLowNotifyThreshold,
+		BalanceLowNotifyRechargeURL:      settings.BalanceLowNotifyRechargeURL,
 	})
 }