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/thank-you.md

@@ -0,0 +1,10 @@
+---
+title: "Thanks for your order"
+description: "Your ebook purchase is complete."
+---
+
+Thanks for your purchase! Check your email in a moment for a link to
+download your book.
+
+If it doesn't show up within a few minutes, check your spam folder, or
+use the [resend form](/books/resend/) to get a fresh link sent to you.