修复整站502(健康探针被慢请求饿死)+全部模特死链+图片并发限流

- 后端 gunicorn 加 --threads 4:纯 sync worker 下几张图就占满 3 worker,
  健康探针 /api/health/ 拿不到 worker→超时→pod 判不就绪→流量被掐→整站502。
  图片生成 I/O 密集,加线程后慢请求不再饿死探针/其他接口。
- 前端图片生成并发上限 2:每张仍独立短请求,但不再 N 条一起砸单 pod 致 OOM。
- ai-tools 全部模特→ 接成展开/收起全部模特(原为纯 span 死链,模特>6 时第7个起选不到)。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/frontend/src/routes/ai-tools.tsx

@@ -490,6 +490,8 @@ export function ImageWorkbenchPage({
   const [gridSort, setGridSort] = useState<"recent" | "name">("recent");
   const [tbOpen, setTbOpen] = useState<"" | "time" | "model">("");
   const [searchOpen, setSearchOpen] = useState(false);
+  // 模特网格默认只露 6 个;「全部模特 →」展开后露全部,再点收起(超过 6 个时第 7 个之后本来选不到)
+  const [showAllModels, setShowAllModels] = useState(false);
   function pickReference(event: ChangeEvent<HTMLInputElement>) {
     const file = event.target.files?.[0];
     if (!file) return;
@@ -834,14 +836,16 @@ export function ImageWorkbenchPage({
                   <div className="iw-step-h">
                     <span className="num">1</span>
                     <span className="title">选择模特</span>
-                    <span className="right">全部模特 →</span>
+                    {personAssets.length > 6 && (
+                      <span className="right" role="button" tabIndex={0} style={{ cursor: "pointer" }} onClick={() => setShowAllModels((v) => !v)} onKeyDown={(e) => { if (e.key === "Enter" || e.key === " ") { e.preventDefault(); setShowAllModels((v) => !v); } }}>{showAllModels ? "收起 ↑" : `全部模特 (${personAssets.length}) →`}</span>
+                    )}
                   </div>
                   <div className="model-grid">
                     {personAssets.length > 0
                       ? personAssets
                           .filter((item) => !gridQuery || item.name.toLowerCase().includes(gridQuery.toLowerCase()))
                           .sort((a, b) => (gridSort === "name" ? a.name.localeCompare(b.name) : (b.created_at || "").localeCompare(a.created_at || "")))
-                          .slice(0, 6)
+                          .slice(0, showAllModels ? undefined : 6)
                           .map((item) => {
                           const url = item.files?.[0]?.preview_url;
                           return (