Permanently delete a loan that was created in error, with its events,
stage entries and servicer row.

Use DELETE /api/partner/loans/{id} instead unless the loan should
never have existed. Archiving is the normal removal and keeps the
history that every historical report reads back through. This endpoint is
for the accident — a loan opened twice, a test record on a live
contact — and it is a separate path precisely so that the archive call
you already make cannot become destructive by adding a parameter.

It refuses anything with history attached, with 422 and a sentence
saying which of these applies:

• the loan mirrors an external record (externalId is set) — that system
  is the authority and a delete here returns on the next sync
• the loan is funded — production and leader reports count it
• credit pulls are attributed to it — they would lose their attribution
• referral partner history references it

Read canDelete and deleteBlockedReason on any loan payload to know the
answer before you call, and to show a person why not.

Publishes loansDeleted, so a subscriber holding a copy of the loan is
told it went away rather than keeping a record that is wrong forever.