Fix stems endpoint with null original filenames

[raymondjacobson]

Summary

• return a stable filename string from the track stems endpoint when tracks.orig_filename is null
• order stems by track ID for deterministic responses
• add a regression case for a stem row with a null original filename

Verification

• go test ./api -run TestGetTrackStems -count=1

Live investigation

• GET /v1/tracks/qJ0RAXE/stems was returning 500: cannot scan NULL into *string for Burko's contest track
• read-only prod query verified parent track 1960897973 has 30 current, non-deleted stem rows, 10 distinct titles, and all 30 rows have orig_filename IS NULL