Add exchange rate feature with multi-currency transactions and transfers support (#1099)

Co-authored-by: Pedro J. Aramburu <pedro@joakin.dev>

lib/money.rb

@@ -39,16 +39,30 @@ class Money
     validate!
   end
 
-  def exchange_to(other_currency, date: Date.current, fallback_rate: nil)
+  # Exchange money to another currency
+  # Params:
+  #   other_currency: target currency code (e.g. "USD")
+  #   date: date for historical rates (default: Date.current)
+  #   custom_rate: explicit exchange rate to use (skips lookup if provided, including nil check)
+  # Priority:
+  #   1. Use custom_rate if explicitly provided (not nil)
+  #   2. Look up rate via store.find_or_fetch_rate
+  #   3. Raise ConversionError if no valid rate available
+  def exchange_to(other_currency, date: Date.current, custom_rate: nil)
     iso_code = currency.iso_code
     other_iso_code = Money::Currency.new(other_currency).iso_code
 
     if iso_code == other_iso_code
       self
     else
-      exchange_rate = store.find_or_fetch_rate(from: iso_code, to: other_iso_code, date: date)&.rate || fallback_rate
+      # Use custom rate if provided, otherwise look it up
+      if custom_rate.present?
+        exchange_rate = custom_rate.to_d
+      else
+        exchange_rate = store.find_or_fetch_rate(from: iso_code, to: other_iso_code, date: date)&.rate
+      end
 
-      raise ConversionError.new(from_currency: iso_code, to_currency: other_iso_code, date: date) unless exchange_rate
+      raise ConversionError.new(from_currency: iso_code, to_currency: other_iso_code, date: date) unless exchange_rate && exchange_rate > 0
 
       Money.new(amount * exchange_rate, other_iso_code)
     end