refactor: 移除 _prefetch_api_dates 内重复 import，补充并发注释

Author: Astro-Han

quantclass_sync_internal/orchestrator.py

@@ -903,9 +903,6 @@ def _prefetch_api_dates(
     已在缓存中且未过期的产品跳过。失败的产品静默跳过，
     Plan 阶段会回退到逐产品 HTTP 查询。
     """
-    import threading
-    from concurrent.futures import as_completed as _as_completed
-
     # 1. 读现有缓存，筛出需要查询的产品
     existing_cache = load_api_latest_dates(log_dir)
     now = datetime.now()
@@ -935,7 +932,8 @@ def _prefetch_api_dates(
         f"[预取] 并发查询 {len(uncached)}/{len(products)} 个产品",
         event="PREFETCH", decision="fetching",
     )
-    fetched: Dict[str, str] = {}
+    fetched: Dict[str, str] = {}  # 写入仅在主线程的 as_completed 循环内，无并发写入
+    # abort_event 只能拦截尚未开始的 worker，已在执行的请求会自然完成或超时
     abort_event = threading.Event()
     t_start = time.time()
 
@@ -958,7 +956,7 @@ def _fetch_one(product: str) -> Tuple[str, Optional[str]]:
     executor = ThreadPoolExecutor(max_workers=effective_workers)
     try:
         futures = {executor.submit(_fetch_one, p): p for p in uncached}
-        for future in _as_completed(futures, timeout=30):
+        for future in as_completed(futures, timeout=30):
             try:
                 product, date_str = future.result()
                 if date_str: