refactor(checkout): move VAT rates, pricing and the PayPal order into pure modules

src/checkout/ holds code without the PayPal SDK or Strapi at runtime, so it can be
unit tested without node_modules:

- vat.ts: vatIncludedDecimal and vatDecimal. config/constants.ts re-exports them
  under the same names and keeps vatDecimalExcluded.
- pricing.ts: calculateTotalProductPrice, unchanged. The product service
  re-exports it, so every importer keeps working.
- paypal-order.ts: the order request createSessionOrThrow builds, in two halves.
  buildPayPalOrderItems (the total guard and the unsent items) still runs
  before the try block, and buildPayPalOrderRequest inside it. Errors and log
  lines stay as they were, for every input. The SDK enums are replaced by their
  string values (CAPTURE, SET_PROVIDED_ADDRESS).

No behaviour change.

Refs libretech/mp#71

config/constants.ts

@@ -1,4 +1,5 @@
 import { Environment } from "@paypal/paypal-server-sdk";
+import { vatDecimal } from "../src/checkout/vat";
 
 export const pdfApiUrl = process.env.PDF_API_ADDRESS!;
 export const mailApiUrl = process.env.MAIL_API_ADDRESS!;
@@ -18,10 +19,7 @@ export const shopEmail = process.env.SHOP_EMAIL ?? process.env.ADMIN_EMAIL_ADDRE
 export const shopLogoUrl = process.env.SHOP_LOGO_URL ?? "";
 export const shopSecondaryLogoUrl = process.env.SHOP_SECONDARY_LOGO_URL ?? "";
 
-// TODO: Should be retrieved from DepotApi
-export const vatIncludedDecimal = 1.19;
-// TODO: Should be retrieved from DepotApi
-export const vatDecimal = 0.19;
+export { vatIncludedDecimal, vatDecimal } from "../src/checkout/vat";
 export const vatDecimalExcluded = 1 - vatDecimal;
 
 export const maxProductsSitemap = 500;