Discord report (CJFC, 2026-04-26): syncing a Spotify playlist to the
server overwrote anything manually added to the server-side playlist.
The fix adds a per-sync mode picker next to the Sync button on the
playlist details modal — Replace (default, current delete-recreate
behavior) or Append only (preserves existing tracks, only adds new
ones). Useful when the source platform caps playlist size and the
user is manually building beyond it on the server.
Implementation:
* New `append_to_playlist(name, tracks)` method on Plex / Jellyfin /
Navidrome clients. Each uses the server's NATIVE append API:
- Plex: `existing_playlist.addItems(new_tracks)`
- Jellyfin: `POST /Playlists/<id>/Items?Ids=...&UserId=...`
- Navidrome: Subsonic `updatePlaylist?songIdToAdd=...`
Falls back to `create_playlist` when the playlist doesn't exist
yet (first sync). No delete-recreate, no backup playlist created
(preserves playlist creation date + metadata + non-soulsync-managed
tracks).
* Dedup-by-server-native-id (ratingKey for Plex, GUID for Jellyfin,
song-id for Navidrome) — never re-adds a track already on the
playlist. Server-native identity, not fuzzy title+artist match,
so it can't false-collide.
* `sync_service.sync_playlist` accepts `sync_mode='replace'|'append'`
kwarg. Single if/else branch dispatches to `append_to_playlist` or
`update_playlist`. Threaded through `core/discovery/sync.run_sync_task`
and the `/api/sync/start` HTTP handler. Validation on the API rejects
unknown mode strings (defaults to 'replace').
* Frontend: per-playlist `<select id="sync-mode-${id}">` rendered next
to the Sync button in both modal renderers (sync-spotify.js for
Spotify playlists, sync-services.js for Deezer ARL playlists).
`startPlaylistSync` reads the select at click time; missing select
(other callers like discover.js) defaults to 'replace' so backward
compat preserved without per-call-site updates.
* SoulSync standalone has no playlist methods at all and the modal
hides the Sync button entirely on it via `_isSoulsyncStandalone` —
dispatch never reaches that path, no defensive fallback needed.
15 new tests pin per-server append behavior:
- missing playlist → create_playlist delegation
- dedup filtering (existing IDs skipped, only new tracks added)
- empty new-track set short-circuits without API call
- failure paths return False without raising
- contract listing (KNOWN_PER_SERVER_METHODS includes
'append_to_playlist'; Plex / Jellyfin / Navidrome all implement)
Plus tests/discovery/test_discovery_sync.py fake `sync_playlist`
fixture got `sync_mode='replace'` default to match the new signature
(was breaking after the kwarg add; now passing).
WHATS_NEW entry under new '2.6.0' block (hidden by
`_getLatestWhatsNewVersion` until next release bump).
Closes CJFC discord request.
