debug: add notification path logging for beta investigation

Add slog.Info/Debug logs to notifyBalanceLow, notifyAccountQuota,
CheckBalanceAfterDeduction, and sendEmails to diagnose why balance
and quota notifications are not being sent in beta environment.

backend/cmd/server/VERSION

@@ -1 +1 @@
-0.1.110.25
+0.1.110.26

backend/internal/service/balance_notify_service.go

@@ -65,14 +65,21 @@ func resolveBalanceThreshold(threshold float64, thresholdType string, totalRecha
 // Notification is sent only on first crossing: oldBalance >= threshold && newBalance < threshold.
 func (s *BalanceNotifyService) CheckBalanceAfterDeduction(ctx context.Context, user *User, oldBalance, cost float64) {
 	if user == nil || s.emailService == nil || s.settingRepo == nil {
+		slog.Debug("CheckBalanceAfterDeduction: skipped (nil check)",
+			"user_nil", user == nil,
+			"email_svc_nil", s.emailService == nil,
+			"setting_repo_nil", s.settingRepo == nil,
+		)
 		return
 	}
 	if !user.BalanceNotifyEnabled {
+		slog.Debug("CheckBalanceAfterDeduction: user notify disabled", "user_id", user.ID)
 		return
 	}
 
 	globalEnabled, globalThreshold := s.getBalanceNotifyConfig(ctx)
 	if !globalEnabled {
+		slog.Info("CheckBalanceAfterDeduction: global notify disabled", "user_id", user.ID)
 		return
 	}
 
@@ -82,18 +89,33 @@ func (s *BalanceNotifyService) CheckBalanceAfterDeduction(ctx context.Context, u
 		threshold = *user.BalanceNotifyThreshold
 	}
 	if threshold <= 0 {
+		slog.Debug("CheckBalanceAfterDeduction: threshold <= 0", "user_id", user.ID, "threshold", threshold)
 		return
 	}
 
 	effectiveThreshold := resolveBalanceThreshold(threshold, user.BalanceNotifyThresholdType, user.TotalRecharged)
 	if effectiveThreshold <= 0 {
+		slog.Debug("CheckBalanceAfterDeduction: effective threshold <= 0", "user_id", user.ID)
 		return
 	}
 
 	newBalance := oldBalance - cost
+	slog.Info("CheckBalanceAfterDeduction: crossing check",
+		"user_id", user.ID,
+		"old_balance", oldBalance,
+		"new_balance", newBalance,
+		"effective_threshold", effectiveThreshold,
+		"crossed", oldBalance >= effectiveThreshold && newBalance < effectiveThreshold,
+	)
 	if oldBalance >= effectiveThreshold && newBalance < effectiveThreshold {
 		siteName := s.getSiteName(ctx)
 		recipients := s.collectBalanceNotifyRecipients(user)
+		slog.Info("CheckBalanceAfterDeduction: sending notification",
+			"user_id", user.ID,
+			"recipients", recipients,
+			"new_balance", newBalance,
+			"threshold", effectiveThreshold,
+		)
 		go func() {
 			defer func() {
 				if r := recover(); r != nil {
@@ -328,11 +350,17 @@ func (s *BalanceNotifyService) collectBalanceNotifyRecipients(user *User) []stri
 
 // sendEmails sends an email to all recipients with shared timeout and error logging.
 func (s *BalanceNotifyService) sendEmails(recipients []string, subject, body string, logAttrs ...any) {
+	if len(recipients) == 0 {
+		slog.Warn("sendEmails: no recipients", "subject", subject)
+		return
+	}
 	for _, to := range recipients {
 		ctx, cancel := context.WithTimeout(context.Background(), emailSendTimeout)
 		if err := s.emailService.SendEmail(ctx, to, subject, body); err != nil {
 			attrs := append([]any{"to", to, "error", err}, logAttrs...)
 			slog.Error("failed to send notification", attrs...)
+		} else {
+			slog.Info("notification email sent successfully", "to", to, "subject", subject)
 		}
 		cancel()
 	}

backend/internal/service/gateway_service.go

@@ -7538,10 +7538,24 @@ func finalizePostUsageBilling(p *postUsageBillingParams, deps *billingDeps, resu
 // to reconstruct oldBalance, avoiding stale Redis reads and concurrent-deduction races.
 func notifyBalanceLow(p *postUsageBillingParams, deps *billingDeps, result *UsageBillingApplyResult) {
 	if p.IsSubscriptionBill || p.Cost.ActualCost <= 0 || p.User == nil || deps.balanceNotifyService == nil {
+		slog.Debug("notifyBalanceLow: skipped",
+			"is_subscription", p.IsSubscriptionBill,
+			"actual_cost", p.Cost.ActualCost,
+			"user_nil", p.User == nil,
+			"service_nil", deps.balanceNotifyService == nil,
+		)
 		return
 	}
 
 	oldBalance := resolveOldBalance(p, result)
+	slog.Info("notifyBalanceLow: calling CheckBalanceAfterDeduction",
+		"user_id", p.User.ID,
+		"old_balance", oldBalance,
+		"cost", p.Cost.ActualCost,
+		"notify_enabled", p.User.BalanceNotifyEnabled,
+		"threshold", p.User.BalanceNotifyThreshold,
+		"result_has_new_balance", result != nil && result.NewBalance != nil,
+	)
 	deps.balanceNotifyService.CheckBalanceAfterDeduction(context.Background(), p.User, oldBalance, p.Cost.ActualCost)
 }
 
@@ -7560,6 +7574,12 @@ func resolveOldBalance(p *postUsageBillingParams, result *UsageBillingApplyResul
 // to avoid a separate DB read that may see stale or concurrently-modified data.
 func notifyAccountQuota(p *postUsageBillingParams, deps *billingDeps, result *UsageBillingApplyResult) {
 	if p.Cost.TotalCost <= 0 || p.Account == nil || !p.Account.IsAPIKeyOrBedrock() || deps.balanceNotifyService == nil {
+		slog.Debug("notifyAccountQuota: skipped",
+			"total_cost", p.Cost.TotalCost,
+			"account_nil", p.Account == nil,
+			"is_apikey_or_bedrock", p.Account != nil && p.Account.IsAPIKeyOrBedrock(),
+			"service_nil", deps.balanceNotifyService == nil,
+		)
 		return
 	}
 	accountCost := p.Cost.TotalCost * p.AccountRateMultiplier
@@ -7567,6 +7587,11 @@ func notifyAccountQuota(p *postUsageBillingParams, deps *billingDeps, result *Us
 	if result != nil {
 		quotaState = result.QuotaState
 	}
+	slog.Info("notifyAccountQuota: calling CheckAccountQuotaAfterIncrement",
+		"account_id", p.Account.ID,
+		"account_cost", accountCost,
+		"has_quota_state", quotaState != nil,
+	)
 	deps.balanceNotifyService.CheckAccountQuotaAfterIncrement(context.Background(), p.Account, accountCost, quotaState)
 }