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/playlists/collage.go

@@ -116,7 +116,7 @@ func loadCellImage(coverPath, dataDir string) image.Image {
 	if err != nil {
 		return fallbackGlyph()
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 	img, _, err := image.Decode(f)
 	if err != nil {
 		return fallbackGlyph()

internal/playlists/collage_test.go

@@ -61,7 +61,7 @@ func TestGenerateCollage_OneTrack(t *testing.T) {
 	if err != nil {
 		t.Fatalf("open collage: %v", err)
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 	img, format, err := image.Decode(f)
 	if err != nil {
 		t.Fatalf("decode collage: %v", err)