feat: Add pluggable bank sync connector architecture

[flaggdavid-source]

Summary

Implements a pluggable architecture for bank integrations as requested in issue #75.

Changes Made

1. Base Connector Interface (app/services/connectors/base.py)

• BaseConnector abstract class with authenticate(), import_transactions(), refresh_accounts() methods
• Transaction and Account dataclasses for data representation
• ConnectorConfig for configuration management
• Exception classes: ConnectorError, AuthenticationError, ImportError, RefreshError

2. Mock Connector (app/services/connectors/mock.py)

• MockConnector implementation for testing and development
• Generates realistic sample transactions
• Configurable via settings (fail_auth, fail_import, simulate_delay, etc.)

3. Connector Registry (app/services/connectors/registry.py)

• ConnectorRegistry factory pattern for dynamic loading
• Pre-registers mock connector
• Convenience functions: get_connector(), list_connectors(), create_connector()

4. Unit Tests (tests/test_connectors.py)

• 29 comprehensive tests covering:
  • Transaction and Account data classes
  • MockConnector authentication and import
  • ConnectorRegistry factory methods
  • Exception handling

Usage Example

from app.services.connectors import create_connector, ConnectorConfig

config = ConnectorConfig(
    connector_id="my-connector",
    connector_type="mock",
    credentials={"api_key": "test-key"}
)
connector = create_connector(config)
connector.authenticate({"api_key": "test-key"})
accounts = connector.refresh_accounts()
transactions = connector.import_transactions(accounts[0].account_id)

All 29 tests pass.