feat(manage): add /manage pages to edit caption and date

/manage lists every photo; /manage/edit/{slug} rewrites the photo's
.toml sidecar, which triggers a rebuild. The pages sit behind the same
htpasswd as /upload and are not linked from the public site.

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

server.go

@@ -35,6 +35,9 @@ func (s *Server) Listen() error {
 	mux := http.NewServeMux()
 	mux.HandleFunc("GET /upload",  s.handleUploadPage)
 	mux.HandleFunc("POST /upload", s.handleUpload)
+	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 /health", func(w http.ResponseWriter, r *http.Request) {
 		fmt.Fprintln(w, "ok")
 	})