[DwC export]: Allow authorized users to build export cache via API

[grantfitzsimmons]

Goal

Expose an API endpoint that allows authorized Institution Admin users to trigger a cache build for a specific export package without using the UI.

Background

Pipeline automation requires scriptable access to the export cache build process. This endpoint is the foundation for cron-based export pipelines. This can be the same as the one used in-app when triggered in #7726

Acceptance Criteria

• A new API endpoint (e.g., just an example, POST /api/exportdataset/{id}/build) triggers a cache build.
• The endpoint requires Institution Admin authentication; non-admins receive 403.
• The endpoint accepts the exportdataset ID as a parameter.
• The response includes a job ID or status URL for polling build progress.
• The endpoint is documented in the Specify API documentation.