fix(HEL-199): 为补档工具补上仓库根 sys.path 引导

使 python3 tools/backfill_recent_snapshots.py --help 在干净环境下可直接运行，并同步文档运行示例为容器内执行。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

tools/backfill_recent_snapshots.py

@@ -12,7 +12,13 @@ from __future__ import annotations
 
 import argparse
 import json
+import sys
 from datetime import date
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+if str(ROOT) not in sys.path:
+    sys.path.insert(0, str(ROOT))
 
 from backend.application import SERVICE
 from backend.bootstrap.config import normalize_date