Repair a transaction journal entry that debits and credits the same account

#2912. For one transaction whose single posted journal entry has both lines on one GL account (balanced, and booking nothing): void-and-replaces it with the canonical two-line posting -- the bank or card offset rebuilt from the transaction's own bank account and direction (money in debits the bank GL, money out credits it), the counterpart on categoryGlAccountId -- and sets the transaction's category to match as a manual decision. The replacement inherits the voided entry's date, period, entity, source, description and posting key; both entries are linked to each other and an audit row records the actor, reason and before/after lines. Dry run by default: it returns the exact before and after lines and writes nothing. An apply (dryRun: false) must name the journal entry the dry run showed. Every eligibility check runs before any write; anything that is not exactly that shape is refused without a guessed replacement. A replay of an applied repair returns already_repaired and writes nothing. Human accountants only: the service token gets 404, and the accountant's assignment is re-read from the database before the request body is parsed.