fix(server): return 404 instead of 500 for non-existent playlists

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.

tests/mock_playlist_repo.go

@@ -8,8 +8,9 @@ import (
 type MockPlaylistRepo struct {
 	model.PlaylistRepository
 
-	Entity *model.Playlist
-	Error  error
+	Entity       *model.Playlist
+	Error        error
+	TracksReturn model.PlaylistTrackRepository
 }
 
 func (m *MockPlaylistRepo) Get(_ string) (*model.Playlist, error) {
@@ -22,6 +23,10 @@ func (m *MockPlaylistRepo) Get(_ string) (*model.Playlist, error) {
 	return m.Entity, nil
 }
 
+func (m *MockPlaylistRepo) Tracks(_ string, _ bool) model.PlaylistTrackRepository {
+	return m.TracksReturn
+}
+
 func (m *MockPlaylistRepo) Count(_ ...rest.QueryOptions) (int64, error) {
 	if m.Error != nil {
 		return 0, m.Error