BankSyncProvider interface

@turbo124 Progress update on the BankSyncProvider interface. I've studied the existing Yodlee + Nordigen implementations and have a draft interface ready for your feedback before I open a full PR. Proposed interface Key design decisions 1. Provider-agnostic transaction shape — the transaction_id field replaces nordigen_transaction_id / Yodlee's transaction_id. Each provider normalizes to this shape via its own transformer. The existing processTransactions() in the job then dedupes on a single column regardless of provider. 2. Status enum alignment — I'm reusing the READY/EXPIRED/SUSPENDED/RATE_LIMITED/ERROR vocabulary the Nordigen job already switches on, so the existing disabled-upstream logic ports cleanly. 3. Authorization flow abstraction — Yodlee is user-account-centric (createUser → getFastTrackUrl), Nordigen is requisition-based (createAgreement → createRequisition → redirect). The interface normalizes this to createAuthorization(institution_id…