Links two reconciliation records into a match. Both records must exist and be reconcilable
with each other; the concrete match subtype is determined automatically from the pair.

Note: the create response returns the bare match, including its type (a persisted STI
column, always present; the list endpoint's methods: [:type] is redundant). Only the embedded
record_a / record_b records are list-only — they are not populated on this create response.