Replace mode (default) deletes + recreates the server playlist every sync,
which wipes its custom image, description, and identity. Add an opt-in
'reconcile' sync mode that edits the existing playlist in place — adds the
tracks now in the source, removes the ones gone — without destroying the
object, so the user's custom art/description survive.
- Pure planner plan_playlist_reconcile(current, desired) -> {add, remove}.
- Per-client reconcile_playlist: Plex addItems/removeItems on the same object;
Navidrome Subsonic updatePlaylist delta (songIdToAdd / descending
songIndexToRemove); Jellyfin add + remove-by-PlaylistItemId on /Playlists/{id}/Items.
- sync_service: reconcile branch with a replace FALLBACK (if a server's in-place
edit is unavailable/fails, sync still succeeds destructively — logged loudly).
- Default stays 'replace' (no behavior change). New Settings > Playlist sync mode
picker (replace/reconcile/append) backed by playlist_sync.mode; per-request
sync_mode still overrides.
- Reconcile skips the post-sync source-image push so a custom poster isn't
re-clobbered (the bug).
Tests: planner (add/remove/dedupe/order/empty) + reconcile-or-replace dispatch
(success / false-fallback / exception-fallback / no-method). Per-server in-place
API calls need dev validation against real Plex/Jellyfin/Navidrome.
NOTE: opt-in only; default behavior unchanged.
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