fix: auto-mark items watched in MDBList on playback completion

[nandhanay]

Fixes #539

When MDBList is selected as the sync provider, completing playback of a movie or episode did not automatically mark the item as watched in MDBList's history, requiring manual marking after every item.

Changes

• MdbListRemoteProvider: On scrobbleStop(), if progress >= WATCHED_THRESHOLD, automatically invoke markMovieWatched / markEpisodeWatched and clear the active playback session.
• Web MDBList Client: On scrobble("stop") with progress >= 80, automatically trigger addToHistory(item).
• Unit Tests: Added MdbListRemoteProviderTest covering movie and episode completion threshold logic.

Testing

• Validated scrobbleStop unit tests for sub-threshold and threshold progress.
• Verified watched history sync behavior.