Learn Classification Defaults

Seed each counterparty's default account from the bank lines already committed to the books, one for each entity whose lines it has: the account most of those lines went to becomes the default, with the lines that agreed counted as confirmations. A default already there is kept. Its still-open lines are then suggested from the default. Run it once on books that were classified before defaults were learned; after that, every commit teaches the default by itself. dry_run reports what would be learned without writing it.

Idempotency: supply an Idempotency-Key header to make safe retries; replays within 24 hours return the same envelope. Reusing the key with a different body returns HTTP 409 Conflict.