[Feature] Audit log for admin actions

Warpgate doesn't record an audit entry for most changes made from the admin side, so there's no way to look back and see who changed what. This matters most for anything that affects access, since the change itself leaves no trace once it's done.

Actions that currently produce no audit entry:

• adding or removing a role from a target
• changing the expiry on an existing role assignment (this can also clear a revocation)
• creating, editing or deleting a role
• creating, editing or deleting a target

It would be good to have an audit entry for each of these, recording the admin who made the change, the object that changed, and the time.