fix: 停止迁移复活默认模型供应商/模型；AI聊天媒体生成改为异步(修复视频请求失败)

- migrations/002_init_data.sql: 移除 model_providers/models 的 INSERT IGNORE 种子，重启容器不再复活 deepseek/openai/通义千问等默认供应商与模型，改由用户在后台大模型库自行维护
- chat.py: 图像/视频生成分支改为立即返回占位(generating=true)，后台 asyncio 任务轮询 AGNES 并回写消息内容，避免最长 5 分钟同步轮询阻塞 HTTP 请求导致前端「发送失败：请求失败」
- ChatPage.tsx: 发送后若 assistant_message.generating，每 3s 轮询 getMessages 直至内容更新；页面刷新时若仍有「⏳」占位继续轮询；卸载清理定时器

frontend/src/pages/front/ChatPage.tsx

@@ -565,8 +565,67 @@ const ChatPage: React.FC = () => {
       }
     };
     loadMessages();
+
+    // 页面刷新时若仍有「⏳」占位的媒体消息，继续轮询后端回写结果
+    mapped.forEach((m) => {
+      if (m.role === 'assistant' && typeof m.content === 'string' && m.content.startsWith('⏳')) {
+        startMediaPolling(id, m.id);
+      }
+    });
   }, [id]);
 
+  // 媒体(图像/视频)异步生成轮询：后端先返回「⏳」占位，前端定时拉取该消息最新内容并回写
+  const pollMediaRef = useRef<Record<string, ReturnType<typeof setInterval> | null>>({});
+
+  const stopMediaPolling = (msgId: string) => {
+    const t = pollMediaRef.current[msgId];
+    if (t) {
+      clearInterval(t);
+      pollMediaRef.current[msgId] = null;
+    }
+  };
+
+  const startMediaPolling = (convId: string, msgId: string) => {
+    if (pollMediaRef.current[msgId]) return; // 已在轮询
+    let ticks = 0;
+    const timer = setInterval(async () => {
+      ticks += 1;
+      try {
+        const res = await chatAPI.getMessages(convId);
+        const msgs: any[] = unwrapList(res);
+        const updated = msgs.find((m: any) => m.id === msgId);
+        if (!updated) {
+          stopMediaPolling(msgId);
+          return;
+        }
+        const content: string = updated.content || '';
+        if (content.startsWith('⏳')) {
+          if (ticks > 120) stopMediaPolling(msgId); // ~6 分钟兜底
+          return;
+        }
+        // 生成完成：回写消息内容
+        setMessages(prev => prev.map(m => m.id === msgId ? {
+          ...m,
+          content,
+          tokenUsed: updated.tokenUsed ?? updated.token_used ?? m.tokenUsed,
+          createdAt: updated.createdAt || updated.created_at || m.createdAt,
+          used_skills: updated.usedSkills ?? updated.used_skills ?? m.used_skills,
+        } : m));
+        stopMediaPolling(msgId);
+      } catch (e) {
+        if (ticks > 120) stopMediaPolling(msgId); // 网络异常兜底
+      }
+    }, 3000);
+    pollMediaRef.current[msgId] = timer;
+  };
+
+  // 卸载时清理所有轮询定时器
+  useEffect(() => {
+    return () => {
+      Object.values(pollMediaRef.current).forEach(t => t && clearInterval(t));
+    };
+  }, []);
+
   // Load knowledge bases, agents, skills for attachment dropdowns
   useEffect(() => {
     knowledgeAPI.getBases().then((res: any) => {
@@ -821,6 +880,11 @@ const ChatPage: React.FC = () => {
         return [...filtered, ...newMsgs];
       });
 
+      // 媒体异步生成：启动轮询，待后端回写后自动刷新
+      if (data.assistant_message?.generating) {
+        startMediaPolling(selectedConversation!.id, data.assistant_message.id);
+      }
+
       // Refresh conversations to update lastMessage
       loadConversations();