feat(manage): delete photos from /manage, prune leftover pages

/manage/delete/{slug} asks for confirmation, then removes the image,
its sidecar and everything generated from them. Builds now drop day
pages of days without photos and pager pages past the last one, so a
delete or a date change leaves nothing behind.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -141,12 +141,16 @@ curl -u user:password \
 
 Or open `https://photos.yourdomain.tld/upload` in a browser.
 
-## Editing a photo
+## Editing and deleting photos
 
 Open `https://photos.yourdomain.tld/manage` in a browser. It lists every
 photo; *Edit* opens a form for the caption and date, and saving rewrites the
 photo's `.toml` sidecar, which triggers a rebuild.
 
+*Delete* asks for confirmation, then removes the image, its sidecar and
+everything generated from them. This is permanent — there is no trash folder,
+so keep a backup of the content directory.
+
 `/manage` sits behind the same htpasswd as `/upload`, and nothing on the
 public site links to it. If you run your own nginx instead of the NixOS
 module, add a `location /manage` block alongside the `/upload` one — without