fix: harden generation task polling fallback

src/api/generationConcurrency.ts

@@ -7,10 +7,20 @@ interface GenerationSlot {
   createdAt: number;
 }
 
-const MAX_ACTIVE_GENERATION_TASKS = 3;
+const DEFAULT_MAX_ACTIVE_GENERATION_TASKS = 3;
 const STALE_SLOT_MS = 6 * 60 * 60 * 1000;
 const activeSlots = new Map<string, GenerationSlot>();
 
+let userMaxConcurrency: number | null = null;
+
+export function setUserMaxConcurrency(limit: number | null | undefined): void {
+  userMaxConcurrency = typeof limit === "number" && limit > 0 ? limit : null;
+}
+
+function getEffectiveLimit(): number {
+  return userMaxConcurrency ?? DEFAULT_MAX_ACTIVE_GENERATION_TASKS;
+}
+
 export function getGenerationUserKey(userId?: string | number | null): string {
   return userId === undefined || userId === null || userId === "" ? "anonymous" : String(userId);
 }
@@ -39,8 +49,9 @@ export function claimGenerationSlot(input: {
 }): () => void {
   pruneStaleSlots();
   const activeCount = getActiveGenerationTaskCount(input.userKey);
-  if (activeCount >= MAX_ACTIVE_GENERATION_TASKS) {
-    throw new Error("当前账号同时最多生成 3 个图片/视频任务，请等待已有任务完成后再提交。");
+  const effectiveLimit = getEffectiveLimit();
+  if (activeCount >= effectiveLimit) {
+    throw new Error(`当前账号同时最多生成 ${effectiveLimit} 个图片/视频任务，请等待已有任务完成后再提交。`);
   }
 
   const id = input.id || `generation-slot-${Date.now()}-${Math.random().toString(36).slice(2, 8)}`;