docs: correct backend stack to PHP/MySQL and document checkout/orders/customers

Swap lingering "Python/MySQL" wording for "PHP / MySQL" across the
README, `src/api/` seam, the Pinia cart store, and the cart contract
doc. Add endpoint specs for checkout (Stripe handoff + webhook),
orders, and customers so the full plug-in surface is documented in
the same style as cart.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/api/cart.js

@@ -1,4 +1,4 @@
-// Cart API — the boundary between the Vue frontend and the Python/MySQL
+// Cart API — the boundary between the Vue frontend and the PHP / MySQL
 // backend. Keep the shapes here stable: everything above this file
 // (components, pages, store consumers) calls these functions and never
 // talks to an endpoint directly.

src/api/index.js

@@ -1,5 +1,5 @@
 // Barrel for the API boundary. Swap these imports for real backend calls
-// when the Python/MySQL side lands — callers keep the same import path.
+// when the PHP / MySQL side lands — callers keep the same import path.
 
 export { products, searchProducts, formatPrice } from './products.js'
 export {

src/api/products.js

@@ -1,4 +1,4 @@
-// Product catalog — frontend fixture until the Python/MySQL backend takes over.
+// Product catalog — frontend fixture until the PHP / MySQL backend takes over.
 // Shape and helper are the same surface the API module will eventually expose.
 
 export const products = [

src/stores/cart.js

@@ -1,5 +1,5 @@
 // Cart state — Pinia store. Persists to localStorage so the basket
-// survives page reloads until the Python/MySQL backend takes over.
+// survives page reloads until the PHP / MySQL backend takes over.
 //
 // Nothing outside this file should import the store directly — the rest
 // of the app goes through `src/api/cart.js`, which is the swap-point