style: gofmt fixes across api package

CI's golangci-lint run flagged three files; two pre-existed this
branch but the Plan 3 seedTrackWithFile struct-literal alignment
is new. Applying gofmt across all three keeps the lint baseline clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/api/library.go

@@ -194,4 +194,3 @@ func (h *handlers) handleListArtists(w http.ResponseWriter, r *http.Request) {
 		Offset: offset,
 	})
 }
-