feat: 模型推理异步加载与对话链路修复，同步基线

模型推理全异步化改造：
- 计算节点 InferenceSession 改为后台线程异步加载模型，load 立即返回，
  加载期间事件循环保持响应（/inference/status 与 /health 不阻塞）
- 后端模型加载改为异步派发 + 轮询对账器（reconcile_inference_loads），
  任务状态由 starting 自动推进到 ready/error，解决多节点启动超时
  （timeout of 120000ms exceeded）
- 推理删除/卸载改为任务感知 + 短超时，删除先删记录再 best-effort 卸载，
  不再被不可达节点阻塞；同节点新模型替换旧任务标记失效
- 流式对话透传 task_id/node_id 路由到真正加载模型的算力节点，
  useStreamChat 解析 SSE 错误帧以干净文案展示
- 对话历史按任务 id 本地持久化，退出重进可恢复；移除页脚提示文本
- 新增后端推理异步加载与计算节点异步状态机单元测试

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/modules/compute_gateway/client.py

@@ -33,6 +33,16 @@ def _unwrap_dict(payload: Any) -> dict[str, Any]:
     return payload if isinstance(payload, dict) else {}
 
 
+# Inference calls are intentionally short-timeout:
+#   - load dispatch only confirms the compute node accepted the request
+#     (the actual model load now runs asynchronously on the node).
+#   - status/unload must never block the platform for long when a node is
+#     unreachable but still marked online.
+INFERENCE_LOAD_TIMEOUT = httpx.Timeout(30, connect=10)
+INFERENCE_STATUS_TIMEOUT = httpx.Timeout(30, connect=5)
+INFERENCE_UNLOAD_TIMEOUT = httpx.Timeout(30, connect=5)
+
+
 class ComputeNodeClient:
     """Application-side client for one compute node.
 
@@ -182,10 +192,16 @@ class ComputeNodeClient:
             response.raise_for_status()
             return _unwrap_dict(response.json())
 
-    async def _request(self, method: str, path: str, json_data: dict[str, Any] | None = None) -> dict[str, Any]:
+    async def _request(
+        self,
+        method: str,
+        path: str,
+        json_data: dict[str, Any] | None = None,
+        timeout: float | None = None,
+    ) -> dict[str, Any]:
         """Generic request method for compute API endpoints."""
         url = _join_url(self.api_base_url, f"{self.route_prefix}{path}")
-        async with httpx.AsyncClient(timeout=300, headers=self.headers()) as client:
+        async with httpx.AsyncClient(timeout=timeout or 300, headers=self.headers()) as client:
             if method.upper() == "GET":
                 response = await client.get(url)
             else:
@@ -193,6 +209,19 @@ class ComputeNodeClient:
             response.raise_for_status()
             return _unwrap_dict(response.json())
 
+    # ── Inference helpers (short timeouts — see module constants) ──────────
+
+    async def inference_load(self, payload: dict[str, Any]) -> dict[str, Any]:
+        """Dispatch a model load. Returns as soon as the node accepts the
+        request; the node now loads asynchronously (status goes 'loading')."""
+        return await self._request("POST", "/inference/load", json_data=payload, timeout=INFERENCE_LOAD_TIMEOUT)
+
+    async def inference_status(self) -> dict[str, Any]:
+        return await self._request("GET", "/inference/status", timeout=INFERENCE_STATUS_TIMEOUT)
+
+    async def inference_unload(self) -> dict[str, Any]:
+        return await self._request("POST", "/inference/unload", json_data={}, timeout=INFERENCE_UNLOAD_TIMEOUT)
+
     async def upload_file(
         self,
         filename: str,