Store and use the update path for a post once it's been created

This way I can keep sending the post to update it in place.

tekuti.el

@@ -109,11 +109,23 @@
                                 (null (alist-get key data nil nil #'string=))
                                 (not (null value)))
                        (insert "#+" key ": " value "\n"))))
-                 args))))
+                 args))
+         (unless (tekuti--get-update-url)
+           (insert "#+UPDATE_URL: "
+                   (string-replace "/posts/"
+                                   "/modify-post/"
+                                   (car (url-path-and-query
+                                         (url-generic-parse-url url))))
+                   "\n"))))
      (message "Created new post: %s" url))
     (`(:error (error http ,code) . ,_)
      (message "Failed to create post: %d" code))))
 
+(defun tekuti--get-update-url ()
+  (save-excursion
+    (goto-char (point-min))
+    (car (map-elt (org-collect-keywords '("UPDATE_URL")) "UPDATE_URL"))))
+
 ;;;###autoload
 (defun tekuti-send ()
   (interactive)
@@ -122,7 +134,9 @@
          (url-request-extra-headers
           `(("Content-Type" . "application/x-www-form-urlencoded")))
          (url-request-data (tekuti-form-encode-data data)))
-    (url-retrieve (format "%s/admin/new-post" tekuti-host)
+    (url-retrieve (format "%s%s" tekuti-host
+                          (or (tekuti--get-update-url)
+                              "/admin/modify-post"))
                   #'tekuti--send-finished
                   (list (current-buffer) data))))