CbeExchangeProvider now parses every currency CBE quotes (USD, DJF, ...)
from the single existing daily-rates fetch instead of hardcoding USD
only, and ExchangeService.getRate gains a pivot step so a pair neither
quoted directly nor as its inverse (e.g. USD->DJF) is derived by
triangulating through the provider's base currency (ETB). Fallback
rates and the load/save callbacks become per-currency instead of a
single USD->ETB scalar. ExchangeService.getRateTable resolves a whole
currency->target rate table in one call for pricing loops.
No behavior change for existing USD/ETB callers.
Claude-Session: https://claude.ai/code/session_01CZy77vCWhka3pnmVF9NDkL
