Rename PlaylistInfo.OwnerUserID to OwnerUsername so plugins provide
usernames instead of internal user IDs, with server-side resolution via
FindByUsername. Fix race condition on playlistGenerators map by using
write lock in startPlaylistGenerators and moving map access inside the
lock in unloadPlugin/Stop. Add context and WaitGroup to the orchestrator
for proper cancellation and goroutine tracking. Include owner_id in the
plugin playlist unique index. Use SetTracks instead of direct assignment
to refresh playlist duration/size/song count stats.
Add a test playlist generator plugin and integration tests for the
PlaylistGenerator capability. The test plugin exercises GetPlaylists and
GetPlaylist WASM functions, and the tests verify orchestration flow
including capability detection, playlist discovery/sync, deterministic
ID generation, error handling, and graceful stop.
Also initialize playlistGenerators map in the test helper to prevent nil
map panics when loading plugins with PlaylistGenerator capability.
