External plan uploads could trigger uniqueness violation on serial PK generator

Background

The addition of activity anchoring means that plans uploaded via the API either need to re-use the existing activity directive IDs as PKs (to preserve anchoring) or have all anchor IDs updated to match new directive IDs in the plan.

If a plan is uploaded with activity directive PKs that are larger than the state of the directive table serial ID generator, then it's possible that a user can encounter a uniqueness violation when trying to add activities to that plan in the future.

Steps to Reproduce

• Create a plan with a single directive whose ID is generated by the serial ID generator in the database. Say that the ID of this directive is 1.
• Via the API, add another directive to this plan, manually setting the ID to 3.
• Via the UI, add one directive to the plan. The generator will assign ID 2 and the directive is created successfully.
• Via the UI, try to add one more directive to the plan. The API call returns a uniqueness violation and the directive is not created.
• Via the UI, try to add one more directive. The generator state has been incremented, so the directive is assigned ID 4 and is created successfully.

Actions

I don't need this resolved as I've already implemented a solution that uses the serial ID generator and re-maps anchored activities; I'm writing this ticket for awareness and documentation.

Start with the API and UI reproduction described in the issue, focusing on the serial ID generator and manually assigned directive IDs. No file, test, entry point, requested change, or completion criterion is provided; the issue says a solution has already been implemented elsewhere.