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/server/server_test.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -20,7 +21,6 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/subsonic"
-	"time"
 )
 
 func TestHealthz(t *testing.T) {
@@ -58,7 +58,7 @@ func TestHealthz_IncludesMinClientVersion(t *testing.T) {
 	if err != nil {
 		t.Fatalf("GET /healthz: %v", err)
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 
 	var body map[string]string
 	if err := json.NewDecoder(resp.Body).Decode(&body); err != nil {
@@ -247,6 +247,6 @@ func TestRouter_AdminSubtreeNotShadowed(t *testing.T) {
 // route-presence assertions in this file.
 type stubScanner struct{}
 
-func (stubScanner) Scan(ctx context.Context) (library.Stats, error) {
+func (stubScanner) Scan(_ context.Context) (library.Stats, error) {
 	panic("stubScanner.Scan called from server_test")
 }