Add database afterRollback callback support and tests

[maltekuhr]

Summary

• add DB::afterRollback to mirror afterCommit
• queue rollback callbacks through the transactions manager
• cover the new API with integration tests

Rationale

• lets applications react immediately after a rollback, e.g. releasing cache locks when a transaction fails
• complements existing afterCommit hooks so both success and failure paths can be handled
• touches only the transaction callback plumbing and facade docblocks, so no behaviour changes for existing code

Testing

• vendor/bin/phpunit tests/Integration/Database/DatabaseTransactionsTest.php