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
+            items.append((name, value))
-        return result
+        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(

tests/test_web_queries.py

@@ -681,6 +681,48 @@ def test_overview_adjacent_previous_with_seed_and_dual_fields():
         finally:
             db.close()
 
+
+def test_system_tasks_sorted_by_started_at_desc():
+    """最近任务按 started_at 倒序；缺开始时间的排末尾（含进行中无 finished）。"""
+    import json
+
+    with tempfile.TemporaryDirectory() as tmp:
+        db, q = setup(tmp)
+        try:
+            repo = Repository(db)
+            # 故意打乱写入/键名字母序；finished_at 与 started_at 错开，避免误用完成时间
+            samples = (
+                # name, started_at, finished_at
+                ("zzz_old", FIXED_NOW - 300, FIXED_NOW - 10),   # 开始最早，完成却很晚
+                ("aaa_mid", FIXED_NOW - 100, FIXED_NOW - 90),
+                ("mmm_new", FIXED_NOW - 10, None),              # 进行中：只有 started
+                ("no_time", None, FIXED_NOW),                  # 缺 started，应末尾
+            )
+            for name, started, finished in samples:
+                payload = {
+                    "name": name,
+                    "ok": True if finished is not None else True,
+                    "error": None,
+                    "summary": {},
+                }
+                if started is not None:
+                    payload["started_at"] = started
+                if finished is not None:
+                    payload["finished_at"] = finished
+                repo.kv_set(f"task:last:{name}", json.dumps(payload))
+            for legacy in ("snapshot", "fans", "discovery"):
+                repo.kv_delete(f"task:last:{legacy}")
+            DASHBOARD_CACHE.clear()
+            system = q.system()
+            assert list(system["tasks"].keys()) == [
+                "mmm_new", "aaa_mid", "zzz_old", "no_time",
+            ]
+            overview = q.overview()
+            assert list(overview["tasks"].keys()) == list(system["tasks"].keys())
+        finally:
+            db.close()
+
+
 def main_test():
     test_overview()
     test_video_listing_filters_and_sorting()
@@ -696,6 +738,7 @@ def main_test():
     test_fans_candidates_system_and_export()
     test_system_status_uses_completed_date_and_running_flags()
     test_system_next_runs_follow_last_completion_not_wall_clock()
+    test_system_tasks_sorted_by_started_at_desc()
     test_growth_auto_falls_back_to_one_day()
     test_snapshot_recent_next_run_follows_completion()
     test_overview_adjacent_previous_with_seed_and_dual_fields()