feat(mileage): 区域与车牌列表级联，并自动剔除越界车牌

- 后端: 选中 region 时基于该区域车辆重算 filters，车牌列表只展示该区域
- 前端: filterOptions.plates 收窄后自动从已选车牌中剔除不属于新区域的项

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/modules/mileage/MonitoringView.tsx

@@ -201,6 +201,14 @@ export default function MonitoringView() {
     loadFirstPage();
   }, [loadFirstPage]);
 
+  // 区域级联：plate 选项收窄后，剔除已选但已不属于该区域的车牌
+  useEffect(() => {
+    if (filterPlates.length === 0) return;
+    const valid = new Set(filterOptions.plates);
+    const next = filterPlates.filter(p => valid.has(p));
+    if (next.length !== filterPlates.length) setFilterPlates(next);
+  }, [filterOptions.plates, filterPlates]);
+
   // 下载当前筛选结果为 xlsx
   const handleDownload = useCallback(async () => {
     if (exporting) return;