Fix clone-assets hanging on large spaces (bounded story-fetch concurrency)

[alpipego]

The clone-assets tool never finishes on spaces with more than a few thousand stories.

Problem

getStories() fetches every story with a single unbounded Promise.all, firing one Management API request per story at once:

const storiesResponsesManagement = await Promise.all(
  links.map((link) =>
    this.targetMapiClient.get(`spaces/${this.targetSpaceId}/stories/${link.id}`)
  )
);

On a large space this instantly exceeds the Management API rate limit and collapses into an endless retry storm, so the migration never reaches the asset-upload step.

Fix

• Fetch stories with async.mapLimit, reusing the existing simultaneousUploads value as the concurrency bound (the upload step already works this way).
• Set an explicit rateLimit on the Management API clients.
• Bump the cdn/links page size to 100 and add per-story progress output.

No interface change; existing configs behave the same, just without the storm.

Verification

Run against a space with 3,941 stories / 1,513 used assets: previously stuck at step 1 with 36k+ rate-limit retries and 0 assets migrated; after the fix it completed end-to-end with 0 retries, 0 failed assets, all 1,513 assets migrated.