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>