feat: 风险分定时衰减 + 密码提示修复 + 浏览器池API + next回跳

1. 风险分衰减定时任务:
   - services/scheduler.py: 每天 CST 04:00 自动执行 decay_scores()
   - 支持 RISK_SCORE_DECAY_TIME_CST 环境变量覆盖

2. 密码长度提示统一为8位:
   - app-frontend/src/pages/RegisterPage.vue
   - app-frontend/src/layouts/AppLayout.vue
   - admin-frontend/src/pages/SettingsPage.vue
   - templates/register.html

3. 浏览器池统计API:
   - GET /yuyx/api/browser_pool/stats
   - 返回 worker 状态、队列等待数等信息
   - browser_pool_worker.py: 增强 get_stats() 方法

4. 登录后支持 next 参数回跳:
   - app-frontend/src/pages/LoginPage.vue: 检查 ?next= 参数
   - 仅允许站内路径（防止开放重定向）

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app-frontend/src/pages/RegisterPage.vue

@@ -4,6 +4,7 @@ import { useRouter } from 'vue-router'
 import { ElMessage } from 'element-plus'
 
 import { fetchEmailVerifyStatus, generateCaptcha, register } from '../api/auth'
+import { validateStrongPassword } from '../utils/password'
 
 const router = useRouter()
 
@@ -68,8 +69,9 @@ async function onSubmit() {
     ElMessage.error(errorText.value)
     return
   }
-  if (password.length < 6) {
-    errorText.value = '密码至少6个字符'
+  const passwordCheck = validateStrongPassword(password)
+  if (!passwordCheck.ok) {
+    errorText.value = passwordCheck.message || '密码格式不正确'
     ElMessage.error(errorText.value)
     return
   }
@@ -166,10 +168,10 @@ onMounted(async () => {
             v-model="form.password"
             type="password"
             show-password
-            placeholder="至少6个字符"
+            placeholder="至少8位且包含字母和数字"
             autocomplete="new-password"
           />
-          <div class="hint app-muted">至少6个字符</div>
+          <div class="hint app-muted">至少8位且包含字母和数字</div>
         </el-form-item>
         <el-form-item label="确认密码 *">
           <el-input