Every caller of Credit/Debit today is a Matrix message, and a Matrix message
arrives once. The Pete games escrow will claim buy-ins over a poll loop, where
a claim that succeeds but whose ack is lost gets retried — and the player pays
twice. euro_transactions had no external id and no unique constraint to stop it.
Adds external_id + a partial unique index, and CreditIdem/DebitIdem: balance
mutation and transaction log in one tx, keyed by the escrow GUID. A replay is a
no-op that still reports ok; a rejection leaves no trace, so the same GUID can
be retried once the player is good for it.
