feat: 新增可配置的路径前缀以支持子路径部署

添加前后端环境变量配置示例，新增后端路径前缀配置项，更新后端路由与静态文件挂载逻辑，调整前端代理配置与 API 调用代码，适配子路径部署场景。

backend/app/static/index.html

@@ -225,6 +225,7 @@ textarea{width:100%;padding:10px;border:1px solid #ddd;border-radius:8px;font-si
 
 <script>
 const API = '';
+const API_PREFIX = window.location.pathname.startsWith('/app_03') ? '/app_03/api' : '/api';
 let TOKEN = '';
 let currentReportId = '';
 let ocrDataItems = [];
@@ -237,6 +238,7 @@ function showLoading(text='加载中...'){
 function hideLoading(){document.getElementById('globalLoading').style.display='none'}
 
 async function api(path,opts={}){
+  if(path.startsWith('/api/')) path = API_PREFIX + path.slice(4);
   const headers = {'Authorization':'Bearer '+TOKEN, ...(opts.headers||{})};
   if(opts.body && !(opts.body instanceof FormData)){
     headers['Content-Type']='application/json';
@@ -270,7 +272,7 @@ function badgeHtml(status){
 async function demoLogin(){
   showLoading('正在登录...');
   try{
-    const data = await fetch(API+'/api/auth/demo-login',{method:'POST'}).then(r=>r.json());
+    const data = await api('/api/auth/demo-login',{method:'POST'});
     TOKEN = data.access_token;
     hideLoading();
   }catch(e){