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/announcements/AnnouncementComposeClient.tsx

@@ -21,6 +21,9 @@ import { ANNOUNCEMENT_COLORS, announcementPublicPath } from "./announcementColor
 type PublishKey = "published" | "draft";
 type StatusFilter = "all" | "published" | "draft";
 
+/** 公告正文字符上限（与后端 service/announcement.go 校验一致） */
+const CONTENT_MAX = 20000;
+
 type FormState = {
   title: string;
   content: string;
@@ -328,8 +331,9 @@ export default function AnnouncementComposeClient({
               value={form.content}
               onChange={(content) => {
                 setErrors((prev) => ({ ...prev, content: undefined }));
-                patchForm({ content });
+                patchForm({ content: content.slice(0, CONTENT_MAX) });
               }}
+              maxChars={CONTENT_MAX}
               allowHide
               allowReplyHide={false}
               placeholder="请输入公告正文，支持 Markdown…"