feat(ai): 后台 AI 链路改走 HTTP 流式，WS 地址与心跳收敛到 utils

写作 / 图片 / 大纲 / 稿件四条链路用普通 POST 时，生成期间链路上零字节，
会被 60s 空闲超时掐成 504（出图实测 58s 断开）。后端已给每条链路开
/stream（SSE，每 15s 一帧心跳），这里改成消费它。

- 新增 utils/ai-stream.ts：SSE 帧解析 + 末帧 {type:'final', result} 拆包；
  runCreativeAssistantStream / runManuscriptGenStream 对外仍返回原来的 data，
  页面渲染逻辑不动
- 新增 utils/chat-socket.ts：buildWebSocketUrl 统一拼 /api/chat/{userId}
  （网关只放行 /api 前缀），attachHeartbeat 每 25s 发 ping 顶住代理空闲超时
- chatgpt 对话先试 WebSocket，握手失败落 /ai/chat/message/stream，
  帧内容与 WS 完全一致，处理函数直接复用
- 四个页面删掉各自重复的 buildWebSocketUrl，统一从 utils 引入

src/views/gxmu/manuscriptGen/index.vue

@@ -229,13 +229,14 @@
   import { asBlob } from 'html-docx-js-typescript';
   import { getProcessor } from 'bytemd';
   import { manuscriptGen } from '@/api/ai/manuscriptGen';
+  import { runManuscriptGenStream } from '@/utils/ai-stream';
   import type {
     ManuscriptGenParams,
     ManuscriptGenResult
   } from '@/api/ai/manuscriptGen/model';
   import { uploadFile } from '@/api/system/file';
   import type { FileRecord } from '@/api/system/file/model';
-  import { API_BASE_URL } from '@/config/setting';
+  import { attachHeartbeat, buildWebSocketUrl } from '@/utils/chat-socket';
   import { useUserStore } from '@/store/modules/user';
   import { copyText } from '@/utils/common';
   import { resolveDisplayText, isFailedPayload, pickDisplayable, FAILED_TEXT } from '@/utils/ai-result';
@@ -560,17 +561,6 @@
     return typeof payload === 'object';
   };
 
-  const buildWebSocketUrl = (userId: number) => {
-    if (/^https?:\/\//.test(API_BASE_URL)) {
-      const url = new URL(API_BASE_URL);
-      url.protocol = url.protocol === 'https:' ? 'wss:' : 'ws:';
-      url.pathname = url.pathname.replace(/\/api\/?$/, '');
-      return `${url.toString().replace(/\/$/, '')}/chat/${userId}`;
-    }
-    const protocol = window.location.protocol === 'https:' ? 'wss:' : 'ws:';
-    return `${protocol}//${window.location.host}/chat/${userId}`;
-  };
-
   const clearStreamResolver = () => {
     streamResolverRef.value = null;
   };
@@ -696,6 +686,7 @@
         }
       };
       socket.onopen = () => {
+        attachHeartbeat(socket);
         wsConnected.value = false;
       };
       socket.onmessage = (event) => {
@@ -859,25 +850,34 @@
     loading.value = true;
     clearGenerated();
     generatedTitle.value = form.theme.trim();
-    responseSummary.value = '正在建立 WebSocket 连接...';
+    responseSummary.value = '正在提交生成任务...';
     try {
-      await connectWebSocket();
+      // 一律走 HTTP 流式（/stream），不再先连 WebSocket：
+      // 触发生成的那个 POST 在整段生成期间是零字节的，会被链路上的 60s 空闲
+      // 超时掐成 504 Gateway Time-out（自由创作、不选文体时最先撞上）。
+      // /stream 每 15s 有后端心跳续命，正文逐段喂给原来的帧处理，
+      // 结构化结果从末帧 {type:'final'} 取。
+      const payload = buildPayload();
       responseSummary.value = templateMode.value
-        ? '连接成功，已提交生成任务，等待流式返回...'
+        ? '已提交生成任务，等待流式返回...'
         : '自由创作已提交给 AI 创意助手，生成完成后一次性返回...';
       const waitStream = createStreamWaiter();
-      const result = await manuscriptGen(buildPayload());
-      if (!streamStarted.value && shouldTreatAsDirectResult(result)) {
+      const result = await runManuscriptGenStream(
+        payload as unknown as Record<string, unknown>,
+        undefined,
+        (raw) => handleSocketMessage(raw)
+      );
+      if (result) {
         applyParsedResult(
-          typeof result === 'string' ? parseSerializedResult(result) : (result as ManuscriptGenResult),
+          typeof result === 'string'
+            ? parseSerializedResult(result)
+            : (result as ManuscriptGenResult),
           '接口直接返回生成结果'
         );
-        loading.value = false;
-        resolveStream();
-        disconnectWebSocket();
-      } else {
-        await waitStream;
       }
+      // 兜底：流结束但没收到 __END__ 时也别把页面卡在加载中
+      resolveStream();
+      await waitStream;
       message.success('内容生成完成');
     } catch (e: any) {
       disconnectWebSocket();