Fix watched-state sync so members' Next Up and Continue Watching follow

Synced rows only ever had the Played flag set. Jellyfin computes Next Up
from LastPlayedDate on the member's own row and Continue Watching from
the resume position, so a member watching through the shared account got
the tick on each episode but their Next Up never advanced. Members are
now written the way BaseItem.MarkPlayed/MarkUnplayed write: date and
position included. Rows the old version ticked without a date are
repaired the next time the item syncs.

PlaybackFinished was also treated as an unwatched toggle. Jellyfin raises
it on every stop, not just completion (and on 10.11 PlaybackStart resets
Played to false first), so a stop halfway through on the shared account
cleared members' own watched state whenever Sync unwatched was on.
Playback-derived reasons (PlaybackFinished, PlaybackProgress,
UpdateUserData) now only ever mirror "watched"; TogglePlayed and Import
remain explicit and mirror either way. PlaybackProgress is acted on so
the tick lands as soon as the completion threshold is crossed, including
for clients that never report a stop.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -141,9 +141,21 @@ Two consequences worth knowing:
 
 ### Watched-state sync
 
-The plugin subscribes to `UserDataSaved` and filters tightly: only `PlaybackFinished`,
-`TogglePlayed` and `Import` are acted on, so the constant stream of progress updates during playback
-is ignored.
+The plugin subscribes to `UserDataSaved` and decides per save reason what it means:
+
+- `TogglePlayed` and `Import` are explicit - someone set the flag - so whatever it says is mirrored,
+  unwatched included (subject to *Sync unwatched*).
+- `PlaybackFinished`, `PlaybackProgress` and `UpdateUserData` only ever mirror *watched*. Jellyfin
+  raises `PlaybackFinished` on every stop, not just on completion, so a stop halfway through leaves
+  `Played` false without anyone having marked anything unwatched; propagating that would wipe what a
+  member watched on their own. Acting on progress too means the tick lands the moment the completion
+  threshold is crossed, and still lands for clients that never report a stop.
+- Everything else (`PlaybackStart`, `UpdateUserRating`) is ignored.
+
+A member is written the way Jellyfin's own *mark played* writes - `Played`, `LastPlayedDate` and a
+cleared resume position - not just the flag. *Next Up* is computed from `LastPlayedDate` on the
+member's own row, so a bare tick would leave their Next Up stuck on the wrong episode. Members whose
+row already matches are skipped, which keeps the progress ticks after the threshold write-free.
 
 No feedback loop is possible: writing to a member raises the event again with *that member's* ID,
 which is not a shared account, so the handler stops immediately.