fix(server): wrap defer tx.Rollback for errcheck + gofmt -s alignment

golangci-lint surfaced both on 9c7dec6:
- errcheck on bare 'defer tx.Rollback(ctx)' in 2 test files
- gofmt -s wanted tighter map-key alignment in library_sync.go +
  library_sync_test.go (auto-fixed by 'gofmt -s -w')

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/library_sync_test.go

@@ -49,7 +49,7 @@ func TestLibrarySync_AfterArtistUpsert_ReturnsHydratedPayload(t *testing.T) {
 	if err != nil {
 		t.Fatalf("begin: %v", err)
 	}
-	defer tx.Rollback(ctx)
+	defer func() { _ = tx.Rollback(ctx) }()
 	var artistID string
 	if err := tx.QueryRow(ctx,
 		`INSERT INTO artists (name, sort_name) VALUES ($1, $2) RETURNING id::text`,
@@ -205,4 +205,3 @@ func TestSplitOnce(t *testing.T) {
 		}
 	}
 }
-