fix: Bank Sync Connector Architecture

[bounty-bot-beep]

Summary

Resolves #75: Bank Sync Connector Architecture

All 36 tests pass (22 existing + 14 new).

Summary of changes for issue #75 (Bank Sync Connector Architecture):

• app/services/bank_sync/ — new pluggable package

  • base.py: BankConnector abstract interface with connect and fetch_transactions, plus BankAccountInfo / BankTransaction dataclasses and ConnectorError
  • registry.py: register_connector decorator, get_connector, list_connectors
  • mock.py: MockConnector self-registers as "mock" with deterministic accounts and transactions
  • __init__.py: re-exports the public surface and eagerly imports the mock

• app/models.py — added BankAccount model (provider, external_id, name, account_type, currency, last_synced_at)

• app/db/schema.sql — added bank_accounts table

• **`app/routes/ba

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Closes #75