feat(mdk-403): add CurrencySchema as single source of truth

Create shared CurrencySchema (z.enum(["USD", "SAT"])) in schemas/currency.ts
and use it throughout the API contract for type safety and validation.

- Add src/schemas/currency.ts with CurrencySchema and Currency type
- Update checkout.ts, products.ts, invoice.ts to use CurrencySchema
- Export CurrencySchema from index.ts

Author: NatElkins

src/contracts/checkout.ts

@@ -1,6 +1,7 @@
 import { oc } from "@orpc/contract";
 import { z } from "zod";
 import { CheckoutSchema } from "../schemas/checkout";
+import { CurrencySchema } from "../schemas/currency";
 
 /**
  * Helper to treat empty strings as undefined (not provided).
@@ -47,7 +48,7 @@ export type CustomerInput = z.infer<typeof CustomerInputSchema>;
 export const CreateCheckoutInputSchema = z.object({
 	nodeId: z.string(),
 	amount: z.number().optional(),
-	currency: z.string().optional(),
+	currency: CurrencySchema.optional(),
 	products: z.array(z.string()).optional(),
 	successUrl: z.string().optional(),
 	allowDiscountCodes: z.boolean().optional(),

src/contracts/products.ts

@@ -1,11 +1,12 @@
 import { oc } from "@orpc/contract";
 import { z } from "zod";
+import { CurrencySchema } from "../schemas/currency";
 
 export const ProductPriceSchema = z.object({
 	id: z.string(),
 	amountType: z.enum(["FIXED", "CUSTOM", "FREE"]),
 	priceAmount: z.number().nullable(),
-	currency: z.string(),
+	currency: CurrencySchema,
 });
 
 export const ProductSchema = z.object({

src/index.ts

@@ -19,6 +19,8 @@ export type {
 } from "./contracts/onboarding";
 export type { Checkout } from "./schemas/checkout";
 export { CheckoutSchema } from "./schemas/checkout";
+export type { Currency } from "./schemas/currency";
+export { CurrencySchema } from "./schemas/currency";
 export type { Product, ProductPrice } from "./contracts/products";
 export {
 	ProductSchema,

src/schemas/checkout.ts

@@ -6,6 +6,7 @@ import {
 	PaidInvoiceSchema,
 } from "./invoice";
 import { CheckoutProductSchema } from "./product";
+import { CurrencySchema } from "./currency";
 
 /**
  * Valid fields that can be required at checkout time.
@@ -42,7 +43,7 @@ const BaseCheckoutSchema = z.object({
 	expiresAt: z.date(),
 	userMetadata: z.record(z.any()).nullable(),
 	customFieldData: z.record(z.any()).nullable(),
-	currency: z.string(),
+	currency: CurrencySchema,
 	allowDiscountCodes: z.boolean(),
 	/**
 	 * Array of customer fields required at checkout.

src/schemas/currency.ts

@@ -0,0 +1,9 @@
+import { z } from "zod";
+
+/**
+ * Supported currencies for pricing and payments.
+ * - USD: US Dollars (amounts in cents)
+ * - SAT: Satoshis (amounts in whole sats)
+ */
+export const CurrencySchema = z.enum(["USD", "SAT"]);
+export type Currency = z.infer<typeof CurrencySchema>;

src/schemas/invoice.ts

@@ -1,12 +1,13 @@
 import { z } from "zod";
+import { CurrencySchema } from "./currency";
 
 export const BaseInvoiceSchema = z.object({
 	invoice: z.string(),
 	expiresAt: z.date(),
 	paymentHash: z.string(),
 	amountSats: z.number().nullable(),
 	amountSatsReceived: z.number().nullable(),
-	currency: z.string(),
+	currency: CurrencySchema,
 	fiatAmount: z.number().nullable(),
 	btcPrice: z.number().nullable(),
 });

src/schemas/product.ts

@@ -1,10 +1,11 @@
 import { z } from "zod";
+import { CurrencySchema } from "./currency";
 
 export const CheckoutProductPriceSchema = z.object({
 	id: z.string(),
 	amountType: z.enum(["FIXED", "CUSTOM", "FREE"]),
 	priceAmount: z.number().nullable(),
-	currency: z.string(),
+	currency: CurrencySchema,
 });
 
 export const CheckoutProductSchema = z.object({