feat(showcase): sort postcards by time descending

AGENTS.md

@@ -0,0 +1,5 @@
+# Project Agents
+
+## 工作习惯
+
+- **文件清理规则**：每次完成任务后，确认所有子任务都完成，再统一删除要清理的文件（如测试脚本、验证脚本），不要边做边删，留到最后一次性处理。

app/routers/web.py

@@ -546,7 +546,11 @@ def public_showcase(request: Request, username: str, db: Session = Depends(get_d
         else:
             p_received = []
         if p_sent or p_received:
+            p_sent.sort(key=lambda c: c.send_time or c.created_at, reverse=True)
+            p_received.sort(key=lambda c: c.receive_time or c.created_at, reverse=True)
             profile_groups.append({"profile": p, "sent": p_sent, "received": p_received})
+    sent_all.sort(key=lambda c: c.send_time or c.created_at, reverse=True)
+    received_all.sort(key=lambda c: c.receive_time or c.created_at, reverse=True)
     return templates.TemplateResponse("showcase.html", {
         "request": request, "profile_user": user, "mode": user.showcase_mode,
         "profile_groups": profile_groups, "sent_all": sent_all, "received_all": received_all,