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/LoginPage.vue

@@ -105,8 +105,14 @@ async function onSubmit() {
       need_captcha: needCaptcha.value,
     })
     ElMessage.success('登录成功，正在跳转...')
+    const urlParams = new URLSearchParams(window.location.search || '')
+    const next = String(urlParams.get('next') || '').trim()
+    const safeNext = next && next.startsWith('/') && !next.startsWith('//') && !next.startsWith('/\\') ? next : ''
     setTimeout(() => {
-      window.location.href = '/app'
+      const target = safeNext || '/app'
+      router.push(target).catch(() => {
+        window.location.href = target
+      })
     }, 300)
   } catch (e) {
     const status = e?.response?.status