feat(api): add GET /api/artists/{id}/tracks handler

Wires ListArtistTracksForUser (Task 3 SQL) to a new handler that returns
all quarantine-filtered tracks for an artist as a flat TrackRef list.
404 when the artist doesn't exist. Route registered in Mount().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/api.go

@@ -43,6 +43,7 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 
 			authed.Get("/artists", h.handleListArtists)
 			authed.Get("/artists/{id}", h.handleGetArtist)
+			authed.Get("/artists/{id}/tracks", h.handleGetArtistTracks)
 			authed.Get("/albums/{id}", h.handleGetAlbum)
 			authed.Get("/albums/{id}/cover", h.handleGetCover)
 			authed.Get("/tracks/{id}", h.handleGetTrack)