Add a [cdn] section and purge the edge after publishing

The site is about to sit behind Cloudflare, where the public pages will be
allowed to live at the edge for a day (`s-maxage`, next commit). That is only
correct if the edge is emptied the moment a new issue lands, so the origin now
does the emptying itself rather than leaving it to an operator to remember.

`[cdn]` is inert by default: with no `provider` nothing is called and no token
is needed, so an origin with no CDN behaves exactly as before. Setting
`provider = "cloudflare"` without both a zone id and `DAILY_EPUB_CDN__API_TOKEN`
is a config error — a half-configured purge would publish into a stale edge and
say nothing.

The purge is `purge_everything` on purpose. A new issue changes more than its
own page: `/`, `/issues`, `/feed.xml`, `/issues.json`, and the previous issue's
page too, whose "latest" nav marker moves. A per-URL list of that set is exactly
the kind of thing that silently rots, and everything expensive at the edge is
content-hashed, so refilling it costs one origin fetch.

A purge failure is logged at warn and never fails the run: the paper is already
published and recorded by then, and a few stale hours are not worth failing over.
`daily-epub cdn purge` runs the same code by hand; it takes no run lock because
it touches neither the database nor the publish directories.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Va5eMEmWEnjMXBsBob5FDW

src/lib.rs

@@ -13,6 +13,7 @@
 //! ```
 
 pub mod auth;
+pub mod cdn;
 pub mod comments;
 pub mod config;
 pub mod curate;