feat: add per-source TTFB and tokens/s metrics to status page

- Req: add FirstByteMs field (ms to first byte, tracked for streaming)
- Stat: add FirstByteSum for aggregation
- SourceAverages(): new method computing per-source avg TTFB and tokens/s
  from the in-memory ring (300s window)
- SourceStatus: add AvgFirstByteMs and AvgTokPerS fields
- pumpStream: record FirstByteMs after first SSE chunk sent to client
- singleChat/singleChatAuto: set FirstByteMs = LatMs (non-streaming)
- handleStatusAPI: populate the new SourceStatus fields from SourceAverages()
- WebUI source table: two new columns showing TTFB (s) and Tokens/s

internal/gateway/ui/index.html

@@ -774,6 +774,8 @@
           tURL: "地址",
           tConn: "连接",
           tConc: "并发",
+          tAvgLat: "首字延迟",
+          tTokSpd: "Token/s",
           online: "在线",
           offline: "退避 / 不可用",
           adTitle: "已加载适配器",
@@ -971,6 +973,8 @@
           tURL: "URL",
           tConn: "Status",
           tConc: "Concurrency",
+          tAvgLat: "TTFB",
+          tTokSpd: "Tokens/s",
           online: "online",
           offline: "backoff / down",
           adTitle: "Loaded adapters",
@@ -1374,7 +1378,9 @@
      <td><b>${esc(x.name)}</b></td><td>${esc(x.adapter)}</td>
      <td><div class="src-models">${(x.models || []).map((m) => `<span class="tag tag-blue modelchip" onclick="showModelConfig('${escAttr(x.name)}','${escAttr(m)}')">${esc(m)}</span>`).join("")}</div></td>
      <td><span class="muted">${esc(x.base_url || "")}</span></td>
-     <td>${x.max_concurrent}</td></tr>`,
+     <td>${x.max_concurrent}</td>
+     <td>${x.avg_first_byte_ms ? (x.avg_first_byte_ms / 1000).toFixed(1) + "s" : "—"}</td>
+     <td>${x.avg_tok_per_s ? x.avg_tok_per_s + " tok/s" : "—"}</td></tr>`,
             )
             .join("");
         } catch (e) {
@@ -1422,7 +1428,7 @@
     ${
       s.sources
         ? `<div class="card"><h2>${t("srcTitle")} (${s.sources.length})</h2>
-      <div class="tbl-wrap"><table><tr><th>${t("tConn")}</th><th>${t("tName")}</th><th>${t("tAdapter")}</th><th>${t("tModels")}</th><th>${t("tURL")}</th><th>${t("tConc")}</th></tr><tbody id="src-body"></tbody></table></div>
+      <div class="tbl-wrap"><table><tr><th>${t("tConn")}</th><th>${t("tName")}</th><th>${t("tAdapter")}</th><th>${t("tModels")}</th><th>${t("tURL")}</th><th>${t("tConc")}</th><th>${t("tAvgLat")}</th><th>${t("tTokSpd")}</th></tr><tbody id="src-body"></tbody></table></div>
       <p class="muted" style="padding:6px 2px 0"><button class="ghost small" onclick="paintSourceStatus()">${t("refreshSrc") || "刷新源状态"}</button> 状态列每 5 秒自动刷新</p>
     </div>`
         : ""