Add Stripe embedded checkout to book pages

Replaces the gumroad_url stub with an on-site embedded checkout (buy
button -> POST to the bookshop backend -> mounted Stripe iframe),
following the existing contact/submissions form's fetch/status-message
pattern. Adds thank-you and resend-link pages, an example book template
(draft), and updates the privacy policy and terms to honestly disclose
Stripe as a third-party payment processor.

content/books/resend.md

@@ -0,0 +1,8 @@
+---
+title: "Resend my download link"
+description: "Lost your download link? Get a new one sent to your email."
+layout: "book-resend"
+---
+
+Enter the email address you used when you bought a book, and if we find
+a matching order we'll send a fresh download link to that address.