fix: the backup package checks or explicitly drops every error the linter flags

Closing a refused S3 response and a test file, and the fake bucket's
writes, now say they ignore the error on purpose; handler parameters the
tests don't use are named _.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/backup/archive_test.go

@@ -154,7 +154,7 @@ func TestWriteFileLandsUnderTheDatedNameOnly(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer f.Close()
+	defer func() { _ = f.Close() }()
 	if _, _, err := Read(f); err != nil {
 		t.Errorf("the written file doesn't read: %v", err)
 	}