feat: 评论/帖子编辑历史与编辑标记，内容锁定

- 新增 smartDiff 差异计算与 EditHistoryModal 编辑历史弹窗，替换原 CommentEditHistoryModal
- 新增 PostEditedMark 编辑标记展示
- 新增内容锁定机制（content_lock）防止并发编辑冲突
- 审核与通知服务适配

frontend/app/admin/content/ContentAdmin.tsx

@@ -25,6 +25,7 @@ import {
   apiAdminContentPosts,
   apiAdminContentComments,
   apiAdminSoftDeletePost,
+  apiCommentHistory,
   apiAdminRestorePost,
   apiAdminPurgePost,
   apiAdminSoftDeleteComment,
@@ -48,7 +49,7 @@ import AutolinkText from "@/components/AutolinkText";
 import ConfirmDialog from "@/components/ConfirmDialog";
 import StaffDeletePostModal from "@/components/StaffDeletePostModal";
 import DeleteCommentModal from "@/components/DeleteCommentModal";
-import CommentEditHistoryModal from "@/components/CommentEditHistoryModal";
+import EditHistoryModal from "@/components/EditHistoryModal";
 import {
   AdminEmpty,
   AdminPageHeader,
@@ -320,6 +321,7 @@ export default function ContentAdmin({
   const [staffDeleteComment, setStaffDeleteComment] = useState<AdminContentComment | null>(null);
   const [staffDeleteCommentBusy, setStaffDeleteCommentBusy] = useState(false);
   const [historyCommentId, setHistoryCommentId] = useState<number | null>(null);
+  const [historyCommentContent, setHistoryCommentContent] = useState("");
 
   const refreshCounts = useCallback(() => {
     apiAdminContentCounts().then(setCounts).catch(() => {});
@@ -710,11 +712,17 @@ export default function ContentAdmin({
                       onReject={() => askReject("comments", c.id)}
                     />
                   ) : null}
-                  <ContentAction
-                    label="历史"
-                    icon={<History size={13} />}
-                    onClick={() => setHistoryCommentId(c.id)}
-                  />
+                  {/* 未编辑（无修订快照）的评论不展示历史入口，避免打开空记录 */}
+                  {c.edited ? (
+                    <ContentAction
+                      label="历史"
+                      icon={<History size={13} />}
+                      onClick={() => {
+                        setHistoryCommentContent(c.content);
+                        setHistoryCommentId(c.id);
+                      }}
+                    />
+                  ) : null}
                   {c.deleted ? (
                     <>
                       {canModerateAuthor({ role: viewerRole }, c.user?.role) ? (
@@ -930,9 +938,14 @@ export default function ContentAdmin({
         }}
       />
 
-      <CommentEditHistoryModal
+      <EditHistoryModal
         open={historyCommentId != null}
-        commentId={historyCommentId}
+        title="评论编辑历史"
+        currentContent={historyCommentContent}
+        fetchPage={(p, s) => {
+          if (historyCommentId == null) return Promise.reject(new Error("未选择评论"));
+          return apiCommentHistory(historyCommentId, p, s);
+        }}
         onClose={() => setHistoryCommentId(null)}
       />
     </div>