chore: replace int amount_cents for string storage and decimal pkg

internal/model/money.go

@@ -0,0 +1,17 @@
+package model
+
+import (
+	"fmt"
+
+	"github.com/shopspring/decimal"
+)
+
+// FormatMoney formats a decimal as a dollar string like "$12.50"
+func FormatMoney(d decimal.Decimal) string {
+	return fmt.Sprintf("$%s", d.StringFixed(2))
+}
+
+// FormatDecimal formats a decimal for form input values like "12.50"
+func FormatDecimal(d decimal.Decimal) string {
+	return d.StringFixed(2)
+}