send Kodi's watched history from a settings button

Not automatically on the service's first start any more: with several Kodi
installs, only the one whose history counts should send it. The button
("Send this Kodi's watched history to the server") asks first, shows progress
and can be cancelled; sending again is harmless, the server keeps newer state.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Readme.md

@@ -13,8 +13,9 @@ versions had it, works too) and an optional auth key, sent as the `Authorization
 
 Watched state and resume points live on the server, shared with other apps (e.g. SKLK TV): listings
 come with them, and a background service (`service.py`) reports playback (every 30 s, on pause, stop
-and end). On its first start the service also sends Kodi's existing watched history of this add-on to
-the server. Mark watched/unwatched with the add-on's own context menu entries ("Mark watched",
+and end). Kodi's own earlier watched history of this add-on can be sent to the server with the button
+in the add-on settings ("Send this Kodi's watched history to the server"); do it on the Kodi whose
+history counts, the server keeps whatever it has that's newer. Mark watched/unwatched with the add-on's own context menu entries ("Mark watched",
 "Mark all watched", ...); Kodi's built-in "Mark as watched" only changes Kodi's local database.
 
 License: no licence