[DwC export]: Support administrator-configured scheduled export pipeline (cron/script)

[grantfitzsimmons]

Goal

Allow an administrator to set up a script or cron job that calls the export cache build API for a specific schema mapping at a configured frequency.

Background

#7741 requires that admins can script export pipeline runs. This issue ensures the endpoint supports the full workflow and documents the approach.

Acceptance Criteria

• The API endpoint from [DwC export]: Allow authorized users to build export cache via API #7741 is fully documented with example curl / script usage.
• The endpoint supports specifying which schema mapping to build.
• The endpoint is stateless and safe to call idempotently (repeated calls without DB changes do no harm).
• Frequency/scheduling can be managed by the caller's cron/task scheduler, not Specify.