The native API endpoints GET /playlist/{id}/tracks and
GET /playlist/{id}/tracks/{id} were panicking with a nil pointer
dereference (resulting in a 500) when the playlist did not exist.
This happened because Tracks() returns nil for missing playlists,
and the nil repository was passed directly to the rest handler.
Extracted a shared playlistTracksHandler that checks for nil and
returns 404 early. Added tests covering both the error and happy paths.
