Allow proposal states to be changed after a decision and let withdrawals be reinstated

[Aqil-Ahmad]

Review decisions on exhibition requests are currently one-way. Once a request leaves
submitted, its state is terminal:

• The organizer's inline/bulk actions (approve / reject / withdraw) only appear while a
  request is submitted. After a decision the row shows no actions, and the detail view
  drops to notes-only.
• A submitter who withdraws their own request cannot undo it, even if it was a mistake.
  There is no "reinstate" action anywhere.

So a misclick is permanent: rejecting the wrong request, or a submitter withdrawing by
accident, can only be undone by editing the database.

This differs from the CfP in orga, where SubmissionStates.valid_next_states makes
decisions reversible — a rejected proposal can be accepted, and a withdrawn one can go
back to submitted.

Notes

• Reactivate, don't duplicate. Withdrawing an accepted request deactivates the
  partner profile it created rather than deleting it. Re-approving must reactivate that
  existing profile — the current approve path returns early when a partner is already
  linked, so it would hand back the deactivated one without re-enabling it.
• Emails. Approve/reject queue the acceptance/rejection emails via the shared model
  methods; a changed decision should queue the email matching the new state, and moving
  back to submitted should not send a decision email.
• Terminal-state guards exist in a few places (list action endpoint, detail review form,
  bulk actions) and all need to move to the shared transition map so the rules stay
  consistent.

Follow-up on #91 #92