Add Changed Fields to Audit Log Entries

auditLog.list entries now include a changedFields map for update actions, exposing per-field [before, after] tuples keyed by field name. Tuple elements ride bare on JSON's own types (string / number / boolean / nested JSON), or — for references to other Ashby objects — wear a { "ref": "<id>", "type": "<objectType>" } sentinel. null on either side means there was no value on that side of the change. The top-level field is null for actions with no per-field changeset (e.g. creates, deletes, or older entries that pre-date the changeset).