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/layouts/AppLayout.vue

@@ -15,6 +15,7 @@ import {
   updateEmailNotify,
 } from '../api/settings'
 import { useUserStore } from '../stores/user'
+import { validateStrongPassword } from '../utils/password'
 
 const route = useRoute()
 const router = useRouter()
@@ -292,8 +293,9 @@ async function onChangePassword() {
     ElMessage.error('请填写完整信息')
     return
   }
-  if (String(newPassword).length < 6) {
-    ElMessage.error('新密码至少6位')
+  const passwordCheck = validateStrongPassword(newPassword)
+  if (!passwordCheck.ok) {
+    ElMessage.error(passwordCheck.message)
     return
   }
   if (newPassword !== confirmPassword) {
@@ -562,7 +564,7 @@ async function dismissAnnouncementPermanently() {
               <el-form-item label="当前密码">
                 <el-input v-model="passwordForm.current_password" type="password" show-password autocomplete="current-password" />
               </el-form-item>
-              <el-form-item label="新密码（至少6位）">
+              <el-form-item label="新密码（至少8位且包含字母和数字）">
                 <el-input v-model="passwordForm.new_password" type="password" show-password autocomplete="new-password" />
               </el-form-item>
               <el-form-item label="确认新密码">