chore(lint): clear pre-existing CI lint debt

Nine golangci-lint failures accumulated across M7 #352, #372, and
6d1709c that were never surfaced because test-go never ran green long
enough to reach the lint step. The first push to land cleanly through
vet (the M7 #363 slice) ran lint and exposed them.

- errcheck: discard the error on defer Close() in collage.go,
  collage_test.go, and server_test.go.
- gofmt -s: re-run on config.go and playlists_test.go.
- goimports: move the stray "time" import from the local-module group
  to stdlib in server_test.go.
- revive unused-parameter: rename ctx to _ on stubScanner.Scan and
  fakeLidarrUnmonitorer.UnmonitorTrack (test stubs); on
  tracks.Service's adminID, add //nolint:revive directive rather than
  renaming because the HTTP handler passes admin.ID (a real authed-user
  UUID) and the existing comment already flags it as reserved for the
  audit-log follow-up.

internal/tracks/service.go

@@ -84,7 +84,7 @@ func NewService(pool *pgxpool.Pool, logger *slog.Logger, lidarr LidarrUnmonitore
 // re-plumb when audit logging lands.
 func (s *Service) RemoveTrack(
 	ctx context.Context,
-	trackID, adminID pgtype.UUID,
+	trackID, adminID pgtype.UUID, //nolint:revive // adminID reserved for audit-log wiring in a follow-up
 	unmonitor bool,
 ) (deletedAlbumID *pgtype.UUID, deletedArtistID *pgtype.UUID, lidarrUnmonitorFailed bool, err error) {
 	q := dbq.New(s.pool)

internal/tracks/service_test.go

@@ -106,7 +106,7 @@ type fakeLidarrUnmonitorer struct {
 	err       error
 }
 
-func (f *fakeLidarrUnmonitorer) UnmonitorTrack(ctx context.Context, trackMbid, albumMbid string) error {
+func (f *fakeLidarrUnmonitorer) UnmonitorTrack(_ context.Context, trackMbid, albumMbid string) error {
 	f.calls = append(f.calls, trackMbid)
 	f.albumMbid = albumMbid
 	return f.err