feat: TTFB 追踪记录与前端展示 + 缓存调度器代码整理

- CLI handler 记录上游请求 TTFB 并注入 proxy_info
- 前端请求时间线展示 TTFB 耗时
- 调度器提升局部 import 到模块顶部，提取 _affinity_hash/_merge_restriction_sets 辅助方法
- 修复缓存亲和性写入的缩进层级
- Python 版本回退至 3.13

src/api/handlers/base/cli_handler_base.py

@@ -932,7 +932,9 @@ class CliMessageHandlerBase(BaseMessageHandler):
                     payload=provider_payload,
                     timeout=request_timeout_sync,
                 )
+                _connect_start = time.monotonic()
                 resp = await http_client.post(**_pkw)
+                ctx.set_ttfb_ms(int((time.monotonic() - _connect_start) * 1000))
             except (httpx.ConnectError, httpx.ConnectTimeout, httpx.TimeoutException) as e:
                 if envelope:
                     envelope.on_connection_error(base_url=ctx.selected_base_url, exc=e)
@@ -971,7 +973,9 @@ class CliMessageHandlerBase(BaseMessageHandler):
                         timeout=request_timeout_sync,
                         refresh_auth=True,
                     )
+                    _connect_start = time.monotonic()
                     resp = await http_client.post(**_pkw)
+                    ctx.set_ttfb_ms(int((time.monotonic() - _connect_start) * 1000))
                     ctx.status_code = resp.status_code
                     ctx.response_headers = dict(resp.headers)
                     if envelope:
@@ -1140,8 +1144,10 @@ class CliMessageHandlerBase(BaseMessageHandler):
                     else None
                 ),
             )
+            _connect_start = time.monotonic()
             response_ctx = http_client.stream(**_skw)
             stream_response = await response_ctx.__aenter__()
+            ctx.set_ttfb_ms(int((time.monotonic() - _connect_start) * 1000))
 
             ctx.status_code = stream_response.status_code
             ctx.response_headers = dict(stream_response.headers)

src/api/handlers/base/stream_context.py

@@ -112,7 +112,7 @@ class StreamContext:
     perf_sampled: bool = False
     perf_metrics: dict[str, Any] = field(default_factory=dict)
 
-    # 代理信息（用于 usage 记录和日志）
+    # 代理信息（用于 usage 记录和日志，含 ttfb_ms）
     proxy_info: dict[str, Any] | None = None
 
     # 流式格式转换状态（跨 chunk 追踪）
@@ -267,6 +267,11 @@ class StreamContext:
         """检查是否因客户端断开连接而结束"""
         return self.status_code == 499
 
+    def set_ttfb_ms(self, ms: int) -> None:
+        """将首字节响应耗时（TTFB）注入到 proxy_info 中"""
+        if self.proxy_info is not None:
+            self.proxy_info["ttfb_ms"] = ms
+
     def build_response_body(self, response_time_ms: int) -> dict[str, Any]:
         """
         构建响应体元数据