fix(notify): per-recipient timeout and return user on email removal

- Use per-recipient context timeout in sendEmails to prevent later
  recipients from failing due to shared timeout exhaustion
- Return updated user object from RemoveNotifyEmail handler for
  frontend state consistency (matching VerifyNotifyEmail pattern)

backend/internal/handler/user_handler.go

@@ -205,5 +205,12 @@ func (h *UserHandler) RemoveNotifyEmail(c *gin.Context) {
 		return
 	}
 
-	response.Success(c, gin.H{"message": "Email removed successfully"})
+	// Return updated user
+	updatedUser, err := h.userService.GetByID(c.Request.Context(), subject.UserID)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	response.Success(c, dto.UserFromService(updatedUser))
 }

backend/internal/service/balance_notify_service.go

@@ -186,13 +186,13 @@ 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) {
-	ctx, cancel := context.WithTimeout(context.Background(), emailSendTimeout)
-	defer cancel()
 	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...)
 		}
+		cancel()
 	}
 }