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/server/routes/mileage/monitoring.ts

@@ -100,6 +100,12 @@ app.get('/', async (c) => {
     filters = buildDateFilters(allVehicles); // 重算筛选选项以匹配权限范围
   }
 
+  // 区域级联：选中运营区域时，下游筛选选项（车牌等）只展示该区域车辆
+  if (filterParams.region) {
+    const regionScope = allVehicles.filter(v => v.region === filterParams.region);
+    filters = buildDateFilters(regionScope);
+  }
+
   const filtered = applyFilters(allVehicles, filterParams);
 
   const stats = {