fix(knowledge): 草稿的过程段写「收件箱 → 待办」，而不是两句「状态变更」

自动生成的草稿里，时间线只查了 activity.action，没带 payload，
于是每条状态变化都变成一模一样的「状态变更」，读者看不出事情怎么走的。

- services/knowledge.snapshot_source 多取 Activity.payload 一起交给渲染
- domain/knowledge 加 STATUS_LABEL 与 _describe()，把 payload 的 from/to 翻成人话
- 新增用例断言草稿里有「待办 → 进行中」「进行中 → 已完成」

66 passed。

backend/app/services/knowledge.py

@@ -26,7 +26,7 @@ async def snapshot_source(session: AsyncSession, ws_id: uuid.UUID, item: Item) -
     )
     timeline = (
         await session.execute(
-            select(Activity.action, Activity.created_at)
+            select(Activity.action, Activity.created_at, Activity.payload)
             .where(Activity.workspace_id == ws_id, Activity.item_id == item.id)
             .order_by(Activity.created_at)
         )
@@ -47,7 +47,7 @@ async def snapshot_source(session: AsyncSession, ws_id: uuid.UUID, item: Item) -
             "提出人": nicknames.get(item.creator_id, ""),
             "办理人": nicknames.get(item.assignee_id, "") if item.assignee_id else "",
         },
-        "timeline": [{"action": a, "at": at} for a, at in timeline],
+        "timeline": [{"action": a, "at": at, "payload": p or {}} for a, at, p in timeline],
         "comments": [
             {"body": body, "author": nicknames.get(author_id, ""), "at": at}
             for body, author_id, at in comments