Currency Rate Update - BNR

This module plugs the Romanian central bank into Odoo's currency rate update system. Once installed, users configuring a currency rate provider can pick "National Bank of Romania" as the service, alongside the other providers already available.

• The provider supports a fixed list of currencies, including EUR, USD, GBP, CHF, RON, PLN, HUF, and others such as XAU and XDR.
• Rates are fetched from the BNR website: a daily file when updating a single day, or a yearly file when updating a date range.
• For each quoted currency, the module converts the published BNR quotation into the rate format Odoo expects, and only keeps currencies the user selected and dates inside the requested period.
• If the daily file returns nothing for a single-day request, it automatically retries using the yearly file.
• On currency rate records, a new stored "inverse rate" field is computed automatically as 1 divided by the rate, so users can see the reverse quotation without entering it manually.

It extends the existing currency rate and currency rate provider models, so it appears wherever those are configured in Odoo's currency settings. No new menus or reports are added.