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>

compute/api/main.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import asyncio
 import json
 import os
 import math
@@ -717,7 +718,9 @@ def create_app() -> FastAPI:
     @app.post(f"{route_prefix}/inference/unload")
     async def inference_unload() -> dict[str, Any]:
         """Unload the currently loaded model and free GPU memory."""
-        return get_inference_session().unload()
+        # Teardown (gc.collect + cuda.empty_cache) can take a while; run it off
+        # the event loop so /health and /inference/status stay responsive.
+        return await asyncio.to_thread(get_inference_session().unload)
 
     @app.get(f"{route_prefix}/inference/status")
     async def inference_status() -> dict[str, Any]:
@@ -737,7 +740,10 @@ def create_app() -> FastAPI:
         messages = payload.get("messages") or []
         if not messages:
             raise HTTPException(status_code=400, detail="messages is required")
-        result = get_inference_session().chat(
+        # Generation is long-running; run it in a thread so the event loop keeps
+        # serving /inference/status and /health during inference.
+        result = await asyncio.to_thread(
+            get_inference_session().chat,
             messages=messages,
             temperature=float(payload.get("temperature", 0.95)),
             top_p=float(payload.get("top_p", 0.7)),