Add exchange_rate column to currencies table

[Haeyzed]

Is your feature request related to a problem? Please describe.
Currently, the currencies table does not store exchange rate information. This makes it difficult to perform currency conversions, pricing normalization, or financial calculations that depend on up-to-date exchange rates within the system.

Describe the solution you'd like
Add an exchange_rate column to the currencies table to store the rate relative to a base currency (e.g. USD). This field can be used for currency conversion, reporting, and financial computations across the application.

Proposed example:

exchange_rate (decimal, nullable or defaulted as needed)

Rate represents conversion against a defined base currency.

Additional context
This change would improve flexibility for multi-currency support and enable easier implementation of pricing, invoicing, and analytics features that depend on exchange rates.

[nnjeim]

@Haeyzed
thank you for your input. The exchange rate is not a constant to be saved in the currency table.
i would see here a relationship where the currency exchange rate is saved in a separate database table, where the latest entry is used in the computation of the pricing you have mentioned needed for billing.

Did i get this right?

[Haeyzed]

@nnjeim

I have a pr for the implementation; if this helps kindly help review to #145

[Haeyzed]

Kindly help review my PR @nnjeim #145