Sets purpose and/or loanTypeKey on a list of loans in one call.

Why this exists. Not every LOS supplies loan purpose or program. When
it does not, imported loans take the default purpose (Purchase) and no
program, and a lender may have hundreds to correct — which is not a job
for one call per loan. GET /api/partner/loans/{id} reports
flags.purposeInferred for a loan whose purpose was defaulted rather
than supplied, so you can find the ones that need it.

Stage is deliberately not settable here. Use
PATCH /api/partner/loans/{id}/stage, so a transition is never a side
effect of a field write.

Ids you cannot see are skipped, not refused. The list is intersected
with what your token may read, and the response reports updated and
skipped so a partially-applicable selection still does the part it can.

Every loan changed emits loansUpdated and records an entry on its
activity history, exactly as a single-loan update does.