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/example-book.md

@@ -0,0 +1,15 @@
+---
+title: "Example Book Title"
+author: "Author Name"
+cover_image: "https://downloads.twosquirrelspress.com/covers/example-book.jpg"
+description: "One-line jacket copy for listing pages."
+stripe_price_id: "price_XXXXXXXXXXXXXXXXXXXXXXXX"
+price_display: "$6.99"
+formats: ["EPUB", "PDF"]
+draft: true
+---
+
+Full jacket-copy / long description goes here. Duplicate this file for
+each real book, fill in the real Stripe Price ID (create the Product in
+the Stripe Dashboard first, with `tsp_book=true` and `file_key=...` set
+in the Product's metadata), and remove `draft: true` to publish it.