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/balance_notify_service.go

@@ -192,11 +192,10 @@ func (s *BalanceNotifyService) getAccountQuotaNotifyEmails(ctx context.Context)
 	var recipients []string
 	seen := make(map[string]bool)
 	for _, entry := range entries {
-		if entry.Disabled {
+		if entry.Disabled || !entry.Verified {
 			continue
 		}
 		email := strings.TrimSpace(entry.Email)
-		// email="" placeholder is not resolved here; admin should configure actual emails
 		if email == "" {
 			continue
 		}
@@ -219,20 +218,17 @@ func (s *BalanceNotifyService) getSiteName(ctx context.Context) string {
 	return name
 }
 
-// collectBalanceNotifyRecipients collects all non-disabled email recipients for balance notifications.
-// Entries with email="" are resolved to the user's primary email.
+// collectBalanceNotifyRecipients returns verified, non-disabled email recipients.
+// Only emails with verified=true and disabled=false are included.
 func (s *BalanceNotifyService) collectBalanceNotifyRecipients(user *User) []string {
 	var recipients []string
 	seen := make(map[string]bool)
 
 	for _, entry := range user.BalanceNotifyExtraEmails {
-		if entry.Disabled {
+		if entry.Disabled || !entry.Verified {
 			continue
 		}
 		email := strings.TrimSpace(entry.Email)
-		if email == "" {
-			email = user.Email // Resolve primary email placeholder
-		}
 		if email == "" {
 			continue
 		}
@@ -244,11 +240,6 @@ func (s *BalanceNotifyService) collectBalanceNotifyRecipients(user *User) []stri
 		recipients = append(recipients, email)
 	}
 
-	// If no entries exist at all (legacy/empty), fall back to user's primary email
-	if len(user.BalanceNotifyExtraEmails) == 0 && user.Email != "" {
-		recipients = append(recipients, user.Email)
-	}
-
 	return recipients
 }