feat(downloads): 多下载器路由:默认下载器、站点指定下载器、别名

照 MP(chain/download/submission.py:明确指定 or 站点下载器 or 默认):
- 迁移 0063:downloader_instance.is_default(至多一个,唯一索引)、site.downloader_instance_id
- pickInstance:明确指定 > 站点指定 > 默认 > 第一个可用;自动下载(gates)与手动下载共用
- 设置:下载器名称改称别名,新增「设为默认下载器」;站点编辑新增「下载器」选择
- 测试 downloader-routing.test.ts

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/server/src/modules/auth-settings/instances-routes.ts

@@ -262,8 +262,15 @@ export function registerInstanceRoutes(app: ModuleApp, ctx: ServerContext) {
       throw new ApiException("validation_failed", "API Key 认证需要填写 API Key");
     return { ...curSettings, ...(next.settings ?? {}), authKind };
   };
+  /** 默认下载器至多一个:设新的之前先取消旧的 */
+  const clearDefault = (exceptId?: string) => {
+    let q = ctx.db.updateTable("downloader_instance").set({ is_default: false }).where("is_default", "=", true);
+    if (exceptId) q = q.where("id", "!=", exceptId);
+    return q.execute();
+  };
   endpoint(app, DL.create, async ({ body, actor }) => {
     const { credential: rawCredential, authKind, ...rest } = body;
+    if (rest.isDefault) await clearDefault();
     const credential = secretInput(rawCredential) ?? undefined;
     const settings = dlAuth(authKind, { ...rest, ...(credential ? { credential } : {}) });
     const keep = authKind !== "none" && credential;
@@ -291,6 +298,7 @@ export function registerInstanceRoutes(app: ModuleApp, ctx: ServerContext) {
     const { credential: rawCredential, authKind, ...rest } = body;
     const credential = secretInput(rawCredential);
     const cur = await getRow("downloader_instance", params.id);
+    if (rest.isDefault) await clearDefault(params.id);
     const curAuth = ((cur.settings ?? {}) as Record<string, unknown>).authKind;
     if (credential === null && (authKind ?? curAuth) === "api_key")
       throw new ApiException("validation_failed", "API Key 认证不能清除 API Key；不用认证时把认证方式改为无认证");