feat(moderation): add user-facing reasons for rejected or failed content

Prompt AI models to provide short explanations for rejected content
Store reasons in database and broadcast via WebSocket
Display moderation details in UI for authors and admins

frontend/src/components/NotificationBell.vue

@@ -296,6 +296,15 @@ function notificationText(notification: NotificationItem) {
   return t('notifications.moderationFailed', { target });
 }
 
+function notificationReasonVisible(notification: NotificationItem) {
+  return (
+    notification.type === 'moderation_result' &&
+    (notification.moderationStatus === 'rejected' || notification.moderationStatus === 'failed') &&
+    notification.moderationReason !== null &&
+    notification.moderationReason.trim() !== ''
+  );
+}
+
 function notificationIcon(notification: NotificationItem) {
   if (notification.type === 'life_post_comment') {
     return iconComment;
@@ -409,6 +418,9 @@ onBeforeUnmount(() => {
             </span>
             <span class="notification-item__copy">
               <strong>{{ notificationText(notification) }}</strong>
+              <span v-if="notificationReasonVisible(notification)" class="notification-item__detail">
+                {{ notification.moderationReason }}
+              </span>
               <time :datetime="notification.createdAt">{{ formatDateTime(notification.createdAt) }}</time>
             </span>
           </button>