sync watched state with the server, single server url setting

- list items take playcount, resume point and last played from the server
  (folders: watched/total episodes), listings aren't cached
- background service (service.py) reports playback to <server>/api/state/<id>:
  every 30 s, on pause, stop and end; on first start it sends Kodi's existing
  watched history of this add-on to the server once
- one "Server URL" setting instead of api + video url (the old api url value,
  ending with /api, still works); no default address

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

Readme.md

@@ -8,4 +8,13 @@ Do **not** try to install a .zip generated by GitHub.
 
 Compatible with Kodi 19.0 ("Matrix") and above that uses Python 3.
 
+Settings: the server URL (e.g. `https://media.example.com`; a value ending with `/api`, as older
+versions had it, works too) and an optional auth key, sent as the `Authorization` header.
+
+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",
+"Mark all watched", ...); Kodi's built-in "Mark as watched" only changes Kodi's local database.
+
 License: no licence