fix(system): sort recent tasks by finished_at desc

「最近任务」原先按 kv 键名排序；改为按 finished_at 倒序，
缺完成时间的排末尾。前端渲染侧同步按 finished_at 排序兜底。

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_finished_at_desc():
+    """最近任务按 finished_at 倒序；缺时间的排末尾。"""
+    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，避免 ORDER BY k 碰巧正确
+            samples = (
+                ("zzz_old", FIXED_NOW - 300),
+                ("aaa_mid", FIXED_NOW - 100),
+                ("mmm_new", FIXED_NOW - 10),
+                ("no_time", None),
+            )
+            for name, finished in samples:
+                payload = {
+                    "name": name,
+                    "ok": True,
+                    "started_at": (finished - 5) if finished is not None else FIXED_NOW,
+                    "error": None,
+                    "summary": {},
+                }
+                if finished is not None:
+                    payload["finished_at"] = finished
+                repo.kv_set(f"task:last:{name}", json.dumps(payload))
+            # 清掉 fixture 里的旧任务，只保留本用例
+            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()
+            # overview 也走 _task_records，顺序应一致
+            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_finished_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()