fix(tools): derive preview task counts from source data

tools/seed_preview_db.py

@@ -254,7 +254,9 @@ def _insert_snapshot(
     )
 
 
-def _seed_task_records(conn: sqlite3.Connection, latest_day: date) -> None:
+def _seed_task_records(
+    conn: sqlite3.Connection, latest_day: date, video_count: int
+) -> None:
     """写入可读的最近任务状态，让预览系统页有内容可看。"""
     base = _timestamp(latest_day, hour=0, minute=1)
     records = {
@@ -264,8 +266,8 @@ def _seed_task_records(conn: sqlite3.Connection, latest_day: date) -> None:
             "finished_at": int(base + 1124),
             "error": None,
             "summary": {
-                "stat_date": latest_day.isoformat(), "total": 796, "ok": 796,
-                "failed": 0, "removed": 0, "completed": True,
+                "stat_date": latest_day.isoformat(), "total": video_count,
+                "ok": video_count, "failed": 0, "removed": 0, "completed": True,
                 "backup_path": f"data/backup/app-{latest_day.strftime('%Y%m%d')}.db",
             },
         },
@@ -560,7 +562,7 @@ def seed_preview(
             up_rows = _seed_up_stats(conn, latest_day, days, anomaly_rng)
             _seed_candidates(conn, latest_day)
             _seed_blacklist(conn, _timestamp(latest_day, hour=14))
-            _seed_task_records(conn, latest_day)
+            _seed_task_records(conn, latest_day, len(selected))
         else:
             up_rows = 0
             conn.execute("DELETE FROM candidate WHERE channel = 'preview'")