Get Slack card image

Get a Slack AI agent card image. Falls back to a placeholder image if the image is not found.

Intentionally public (no auth): Slack's unfurl bot fetches preview images without user credentials. Access is guarded by an unguessable 21-char nanoid (only minted server-side and shared with Slack during the unfurl of a resource the user already has access to), strict {id} validation, and no-store / noindex response headers.