Adds the schema scaffolding for the fourth approval action (alongside
Approve / Reject / Revoke):
1. Extends approval_requests.status CHECK to include 'changes_requested'.
2. Adds counter_payload jsonb — the approver's edited values on a
changes_requested row (the basis of the new row's payload).
3. Adds previous_request_id uuid FK — back-pointer from a SuggestChanges-
spawned row to its source. Partial index on the FK supports chain
traversal.
Non-blocking: extending a CHECK constraint is metadata-only on Postgres;
adding NULLable columns + a NULLable FK is metadata-only. Safe for live
deploy.
Dry-run validated against the live youpc paliad schema via BEGIN/ROLLBACK
(migration tracker at 102 pre-apply; schema unchanged post-rollback).
