Test the checkout code; publish images only for releases (#5)

- src/checkout/ (PayPal order request, pricing, VAT) and src/logging/format.ts
  hold the checkout's pure code, moved out of PayPalApi, the product service
  and config without a behaviour change.
- npm test runs node:test on the TypeScript sources (type stripping), with no
  dependencies: 21 tests, 13 pin today's output, 8 describe the correct
  behaviour for the known defects of libretech/mp#67
  and are marked todo.
- CI: the tests run in the image's base (node:22.14.0-alpine) for every pull
  request, push to main and release tag. The image is built for pull requests
  and built and published only for v* tags; main no longer publishes :main
  and :sha-* images.

Refs libretech/mp#71

Co-authored-by: Michael Czechowski <mail@librete.ch>

tests/unit/pricing.test.ts

@@ -0,0 +1,26 @@
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { calculateTotalProductPrice } from "../../src/checkout/pricing.ts";
+
+// Characterization: what calculateTotalProductPrice returns today.
+
+test("adds up the cover, pages and ruling prices", () => {
+	assert.equal(calculateTotalProductPrice({ cover: { price: 14.9 }, pages: { price: 3 }, ruling: { price: 0.5 } }), 18.4);
+});
+
+test("prices a standalone product from its cover alone", () => {
+	assert.equal(calculateTotalProductPrice({ cover: { price: 9.5 } }), 9.5);
+});
+
+test("counts a missing component or price as 0", () => {
+	assert.equal(calculateTotalProductPrice({ cover: null, pages: { price: null }, ruling: undefined }), 0);
+});
+
+test("returns 0 for a missing product instead of throwing", () => {
+	assert.equal(calculateTotalProductPrice(null), 0);
+	assert.equal(calculateTotalProductPrice(undefined), 0);
+});
+
+test("does not round the sum", () => {
+	assert.equal(calculateTotalProductPrice({ cover: { price: 0.1 }, pages: { price: 0.2 } }), 0.30000000000000004);
+});