Watch state is merged from several sources

Kustos reads watch state from Plex history, Tautulli, Trakt and Simkl and merges them into one picture. That matters because:

• A gap in one source (a missed scrobble, Tautulli downtime) can't drop a title someone is still watching.
• One source works. Two is more reliable.
• Movies work on any single source. TV per-season needs per-episode data, which Plex history (if enabled), Tautulli and Trakt all provide.

The completion threshold ("what counts as watched") is a setting; it sits below 100% so finishing during long end credits still counts.