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>

resources/settings.xml

@@ -1,6 +1,6 @@
-<?xml version="1.0" encoding="utf-8" standalone="yes"?>
-<settings>
-    <setting id="api_url" type="text" default="http://sklk.lt:8181/api" label="API URL"/>
-    <setting id="video_url" type="text" default="http://sklk.lt:8181" label="VIDEO URL"/>
-    <setting id="auth_key" type="text" default="" label="Auth key"/>
-</settings>
+<?xml version="1.0" encoding="utf-8" standalone="yes"?>
+<settings>
+    <!-- the server, e.g. https://media.example.com ("api_url" for older versions' value, which ended with /api) -->
+    <setting id="api_url" type="text" default="" label="Server URL"/>
+    <setting id="auth_key" type="text" default="" label="Auth key"/>
+</settings>