Add pluggable bank connector architecture

[Entr0zy]

Closes #75

Implements a pluggable backend bank-sync connector foundation:

• Adds a BankConnector interface for provider integrations.
• Adds ConnectorRegistry for connector lookup and registration.
• Adds MockBankConnector as the included mock connector.
• Adds import_bank_transactions(...) and refresh_bank_transactions(...) helpers that normalize provider rows into FinMind's existing import transaction shape.
• Covers registry lookup, import date windows, refresh support, normalization, and validation errors with unit tests.

Validation:

• python -m pytest tests/test_bank_connectors.py -q -> 5 passed
• python -m py_compile app/services/bank_connectors.py tests/test_bank_connectors.py -> passed
• git diff --check -> passed

Additional note:

• python -m pytest tests/test_expenses.py tests/test_bank_connectors.py -q runs the new connector tests but the existing expense tests error in this local environment while attempting to connect to redis:6379 during auth setup. The failure happens before the bank connector code is exercised.