fix: unify backend port to 18080 and make validator targets configurable

- Set default API port to 18080 in config.py
- Add configurable validation_targets to SettingsSchema and DEFAULT_SETTINGS
- Update ValidatorService to support runtime test URL updates
- Hot-reload validation_targets from DB on startup and on settings save
- Add domestic fallback URLs (baidu.com, qq.com) to reduce foreign dependency risk
- Update Settings.vue to allow adding/removing validator target URLs in UI

WebUI/src/views/Settings.vue

@@ -130,6 +130,27 @@
           />
         </el-form-item>
 
+        <el-form-item label="验证目标地址" prop="validation_targets">
+          <el-select
+            v-model="settings.validation_targets"
+            multiple
+            filterable
+            allow-create
+            default-first-option
+            placeholder="输入并按回车添加验证 URL"
+            class="setting-input"
+            style="width: 500px;"
+          >
+            <el-option
+              v-for="url in defaultValidationTargets"
+              :key="url"
+              :label="url"
+              :value="url"
+            />
+          </el-select>
+          <span class="setting-hint">代理验证时将随机轮询这些地址，建议包含多个国内外站点</span>
+        </el-form-item>
+
         <el-form-item label="自动验证间隔" prop="validate_interval_minutes">
           <el-input-number 
             v-model="settings.validate_interval_minutes" 
@@ -218,9 +239,19 @@ const settings = reactive({
   min_proxy_score: 0,
   proxy_expiry_days: 7,
   auto_validate: true,
-  validate_interval_minutes: 30
+  validate_interval_minutes: 30,
+  validation_targets: []
 })
 
+const defaultValidationTargets = [
+  'http://httpbin.org/ip',
+  'https://httpbin.org/ip',
+  'http://api.ipify.org',
+  'https://api.ipify.org',
+  'http://www.baidu.com',
+  'http://www.qq.com'
+]
+
 // ==================== 计算属性 ====================
 const schedulerInfo = computed(() => {
   if (schedulerRunning.value) {