refactor: BankingOGM::preSave() uses serial module for sequence generation
Replaces the manual SELECT MAX(...)+1 query with the serial contrib module's atomic counter service (serial.sql_storage), matching the pattern already used by shareholder_register. The per-bundle prefix formatting is preserved, but the underlying sequence number is now generated via an atomic auto-increment instead of a racy max+1 query.
Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com