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

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

app/controllers/exchange_rates_controller.rb

@@ -0,0 +1,36 @@
+class ExchangeRatesController < ApplicationController
+  def show
+    # Pure currency-to-currency exchange rate lookup
+    unless params[:from].present? && params[:to].present?
+      return render json: { error: "from and to currencies are required" }, status: :bad_request
+    end
+
+    from_currency = params[:from].upcase
+    to_currency = params[:to].upcase
+
+    # Same currency returns 1.0
+    if from_currency == to_currency
+      return render json: { rate: 1.0, same_currency: true }
+    end
+
+    # Parse date
+    begin
+      date = params[:date].present? ? Date.parse(params[:date]) : Date.current
+    rescue ArgumentError, TypeError
+      return render json: { error: "Invalid date format" }, status: :bad_request
+    end
+
+    begin
+      rate_obj = ExchangeRate.find_or_fetch_rate(from: from_currency, to: to_currency, date: date)
+    rescue StandardError
+      return render json: { error: "Failed to fetch exchange rate" }, status: :bad_request
+    end
+
+    if rate_obj.nil?
+      return render json: { error: "Exchange rate not found" }, status: :not_found
+    end
+
+    rate_value = rate_obj.is_a?(Numeric) ? rate_obj : rate_obj.rate
+    render json: { rate: rate_value.to_f }
+  end
+end