Backend now stores user settings (dashboard config) in a JSONB column and
exposes CRUD via /settings/. Exchange rate service gains multiple fallback
providers (ExchangeRate-API, currency-api, FloatRates) so USD/CRC rates
stay available when BCCR is down.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>