Add bank sync connector architecture

[sayuru-akash]

Summary

• add a pluggable bank connector interface and registry with a built-in mock connector
• add bank connection import and refresh endpoints with sync-run tracking and external transaction dedupe
• add schema/runtime compatibility tables, OpenAPI docs, README notes, and Redis fallback resilience for local/dev auth and cache flows

Bounty

Addresses #75 (Bank Sync Connector Architecture, bounty).

Tests

• PYTHONPATH=. .venv311/bin/pytest tests
• .venv311/bin/black --check app tests
• .venv311/bin/flake8 app tests

[sayuru-akash]

Update pushed to better match the Indian AA/API-provider requirement from the issue discussion.\n\nAdded a generic account_aggregator connector alongside mock so provider-specific adapters can store provider/account/consent references and import normalized partner-portal transaction payloads without putting live credentials in app config. Updated OpenAPI/README and added coverage for the normalized AA payload path.\n\nVerification after the update:\n- PYTHONPATH=. .venv311/bin/pytest tests -> 29 passed\n- .venv311/bin/black --check app tests -> passed\n- .venv311/bin/flake8 app tests -> passed\n- git diff --check -> passed