fix: add nolint:errcheck comments for intentionally unchecked errors

Add //nolint:errcheck comments to intentionally unchecked error returns:

- defer X.Close() calls: errors from closing read-only resources, rows
  after iteration, files, response bodies, and gzip readers are not
  actionable and don't affect correctness

- defer tx.Rollback(): standard Go pattern where rollback after
  successful commit returns an error, which is expected behavior

- defer stmt.Close(): statements are closed with their transactions

- Cleanup operations: DeleteRevision on failure and os.RemoveAll for
  temp directories are best-effort cleanup

- HTTP response encoding: if JSON encoding fails at response time,
  there's nothing useful we can do

- Test/benchmark code: unchecked errors in test setup/cleanup where
  failures will surface through test assertions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/nixos/indexer_test.go

@@ -77,7 +77,7 @@ func BenchmarkIndexRevision(b *testing.B) {
 	if err != nil {
 		b.Fatalf("Failed to create store: %v", err)
 	}
-	defer store.Close()
+	defer store.Close() //nolint:errcheck // benchmark/test cleanup
 
 	ctx := context.Background()
 	if err := store.Initialize(ctx); err != nil {
@@ -90,7 +90,7 @@ func BenchmarkIndexRevision(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		// Delete any existing revision first (for repeated runs)
 		if rev, _ := store.GetRevision(ctx, TestNixpkgsRevision); rev != nil {
-			store.DeleteRevision(ctx, rev.ID)
+			_ = store.DeleteRevision(ctx, rev.ID) //nolint:errcheck // benchmark cleanup
 		}
 
 		result, err := indexer.IndexRevision(ctx, TestNixpkgsRevision)
@@ -119,7 +119,7 @@ func BenchmarkIndexRevisionWithFiles(b *testing.B) {
 	if err != nil {
 		b.Fatalf("Failed to create store: %v", err)
 	}
-	defer store.Close()
+	defer store.Close() //nolint:errcheck // benchmark/test cleanup
 
 	ctx := context.Background()
 	if err := store.Initialize(ctx); err != nil {
@@ -132,7 +132,7 @@ func BenchmarkIndexRevisionWithFiles(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		// Delete any existing revision first
 		if rev, _ := store.GetRevision(ctx, TestNixpkgsRevision); rev != nil {
-			store.DeleteRevision(ctx, rev.ID)
+			_ = store.DeleteRevision(ctx, rev.ID) //nolint:errcheck // benchmark cleanup
 		}
 
 		result, err := indexer.IndexRevision(ctx, TestNixpkgsRevision)
@@ -168,7 +168,7 @@ func BenchmarkIndexFilesOnly(b *testing.B) {
 	if err != nil {
 		b.Fatalf("Failed to create store: %v", err)
 	}
-	defer store.Close()
+	defer store.Close() //nolint:errcheck // benchmark/test cleanup
 
 	ctx := context.Background()
 	if err := store.Initialize(ctx); err != nil {
@@ -211,7 +211,7 @@ func TestIndexRevision(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to create store: %v", err)
 	}
-	defer store.Close()
+	defer store.Close() //nolint:errcheck // benchmark/test cleanup
 
 	ctx := context.Background()
 	if err := store.Initialize(ctx); err != nil {