fix(notify): use real-time balance for crossing detection and simplify email logic

- Fix cached balance causing threshold crossing to never trigger:
  read real-time balance from billingCacheService instead of stale
  API key auth snapshot
- Remove email="" placeholder concept; all emails are user-managed
- Only send notifications to verified && non-disabled emails
- Frontend: pre-fill user's email in add input when list is empty
- Remove FilterEnabledEmails/IsPrimaryDisabled helpers (no longer needed)

backend/internal/service/gateway_service.go

@@ -7338,9 +7338,15 @@ func finalizePostUsageBilling(p *postUsageBillingParams, deps *billingDeps) {
 
 	deps.deferredService.ScheduleLastUsedUpdate(p.Account.ID)
 
-	// Balance low notification
+	// Balance low notification — use real-time balance from billing cache (not stale snapshot)
 	if !p.IsSubscriptionBill && p.Cost.ActualCost > 0 && p.User != nil && deps.balanceNotifyService != nil {
-		deps.balanceNotifyService.CheckBalanceAfterDeduction(context.Background(), p.User, p.User.Balance, p.Cost.ActualCost)
+		oldBalance := p.User.Balance // fallback to snapshot
+		if deps.billingCacheService != nil {
+			if realBalance, err := deps.billingCacheService.GetUserBalance(context.Background(), p.User.ID); err == nil {
+				oldBalance = realBalance + p.Cost.ActualCost // DB already deducted, reconstruct pre-deduction balance
+			}
+		}
+		deps.balanceNotifyService.CheckBalanceAfterDeduction(context.Background(), p.User, oldBalance, p.Cost.ActualCost)
 	}
 
 	// Account quota notification (use same cost formula as postUsageBilling)