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>

server.go

@@ -38,6 +38,8 @@ func (s *Server) Listen() error {
 	mux.HandleFunc("GET /manage", s.handleManagePage)
 	mux.HandleFunc("GET /manage/edit/{slug}", s.handleEditPage)
 	mux.HandleFunc("POST /manage/edit/{slug}", s.handleEdit)
+	mux.HandleFunc("GET /manage/delete/{slug}", s.handleDeletePage)
+	mux.HandleFunc("POST /manage/delete/{slug}", s.handleDelete)
 	mux.HandleFunc("GET /health", func(w http.ResponseWriter, r *http.Request) {
 		fmt.Fprintln(w, "ok")
 	})