feat(fusion_accounting_reports): currency conversion service

Pure-Python helper for FX conversion at report end-date. Handles direct
rates, inverse rates, and fallback to most-recent-rate-on-or-before.
fetch_rates() pulls from res.currency.rate using the same
1/rate inversion convention Odoo uses internally.

Made-with: Cursor

fusion_accounting_reports/services/__init__.py

@@ -1,3 +1,4 @@
 from . import date_periods
 from . import account_hierarchy
 from . import totaling
+from . import currency_conversion