fix: 删除 routers/websites 中残留的死代码函数 _build_rate_priority_map，并为 build_target_group_priority_map 引入严格的分组倍率解析，防止非法/缺失倍率的分组以 1.0 的降级值参与排序

backend/app/services/website_sync.py

@@ -220,6 +220,17 @@ def _snapshot_group_rate(group: dict) -> float:
         return 1.0
 
 
+def _strict_snapshot_group_rate(group: dict) -> float | None:
+    """从快照分组数据中严格提取倍率，若缺失或非数值则返回 None。"""
+    raw = group.get("rate") or group.get("default_rate") or group.get("rate_multiplier")
+    if raw is None or raw == "":
+        return None
+    try:
+        return float(raw)
+    except (TypeError, ValueError):
+        return None
+
+
 def build_rate_priority_map(db: Session, upstream_ids: set[int]) -> dict[str, int]:
     """根据上游分组倍率构建 f"{upstream_id}:{group_id}" → priority 映射。
 
@@ -465,8 +476,9 @@ def build_target_group_priority_map(
             groups = rate_maps.get(upstream_id) or {}
             g = groups.get(group_id)
             if isinstance(g, dict):
-                rate = _snapshot_group_rate(g)
-                rated_sources.append(((upstream_id, group_id), rate))
+                rate = _strict_snapshot_group_rate(g)
+                if rate is not None:
+                    rated_sources.append(((upstream_id, group_id), rate))
 
         if not rated_sources:
             continue