Class CustomerBankPartnerService

Editor-facing split view: the customer's explicitly-stored partners and the DB-driven defaults, kept separate rather than merged.

The merged get view is wrong to edit against: it folds defaults into the list, so a naive get→save round-trip would freeze today's inherited defaults as explicit per-customer picks (and they'd stop tracking future default changes). An editor writes back only the stored set via upsert; the defaults are presented as read-only context so the operator sees what an empty override inherits. assignable is every partner bank the schema permits that also exists in the bank table (enriched with name/routing) — the only banks upsert will accept without toBankPartner throwing.

Create or update the customer's stored bank partners.

The stored list is replaced wholesale with data (the partner ids + default overrides only; name/routingNumber are enrichment and are not persisted). Returns the enriched, merged-with-defaults view.