fix(system): sort recent tasks by started_at desc

「最近任务」原先按 kv 键名排序。改为后端 `_task_records` 按
started_at 倒序写入 dict（缺开始时间排末尾，含进行中）；
前端不再参与排序。配套 test_system_tasks_sorted_by_started_at_desc。

reaction_board/web/queries.py

@@ -622,19 +622,32 @@ class DashboardQueries:
                 continue
         return None
 
+    @staticmethod
+    def _task_started_sort_key(value: Any) -> tuple[int, int]:
+        """缺 started_at 的排末尾；有值的按开始时间倒序（含进行中）。"""
+        if isinstance(value, Mapping):
+            raw = value.get("started_at")
+            try:
+                return (0, -int(raw))
+            except (TypeError, ValueError):
+                pass
+        return (1, 0)
+
     def _task_records(self, conn: sqlite3.Connection) -> dict[str, Any]:
         rows = conn.execute(
-            "SELECT k, v FROM kv WHERE k LIKE 'task:last:%' ORDER BY k"
+            "SELECT k, v FROM kv WHERE k LIKE 'task:last:%'"
         ).fetchall()
-        result: dict[str, Any] = {}
+        items: list[tuple[str, Any]] = []
         for row in rows:
             name = str(row["k"]).removeprefix("task:last:")
             try:
                 value = json.loads(str(row["v"]))
             except (TypeError, json.JSONDecodeError):
                 value = {"raw": row["v"]}
-            result[name] = value
-        return result
+            items.append((name, value))
+        items.sort(key=lambda pair: self._task_started_sort_key(pair[1]))
+        # 插入序即开始时间倒序；前端 Object.entries 保留该序，勿再排序。
+        return dict(items)
 
     def _task_running(self, conn: sqlite3.Connection) -> dict[str, Any]:
         rows = conn.execute(