fix: 最新公告直接返回最新5条 + 显示来源网站 + 菜单调整

- latest_announcements 不再过滤 keyword_matched，返回全部最新5条
- 每条公告显示来源网站（广西政府采购网/大化县政府网采购公告）
- 菜单「今日公告」改为顶级按钮「最新公告」，移除子菜单重复项

app/wechat/menu.py

@@ -9,9 +9,9 @@ logger = logging.getLogger(__name__)
 MENU = {
     "button": [
         {
-            "name": "今日公告",
+            "name": "最新公告",
             "type": "click",
-            "key": "today_stats",
+            "key": "latest_announcements",
         },
         {
             "name": "查询",
@@ -31,11 +31,6 @@ MENU = {
                     "type": "click",
                     "key": "workday_status",
                 },
-                {
-                    "name": "最新公告",
-                    "type": "click",
-                    "key": "latest_announcements",
-                },
             ],
         },
         {