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)
 	}

internal/backup/s3.go

@@ -86,7 +86,7 @@ func (b Bucket) do(req *http.Request, payloadSHA256 string) (*http.Response, err
 		return nil, fmt.Errorf("%w: %s is unreachable: %v", ErrS3, req.URL.Host, err)
 	}
 	if resp.StatusCode/100 != 2 {
-		defer resp.Body.Close()
+		defer func() { _ = resp.Body.Close() }()
 		return nil, refusal(resp)
 	}
 	return resp, nil

internal/backup/s3_test.go

@@ -77,7 +77,7 @@ func testBucket(url string) Bucket {
 func TestPutSendsTheBodyToItsPathStyleKey(t *testing.T) {
 	var gotPath, gotSHA, gotBody, gotAuth string
 	var gotLen int64
-	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	srv := httptest.NewServer(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
 		gotPath, gotSHA, gotAuth, gotLen = r.URL.EscapedPath(), r.Header.Get("X-Amz-Content-Sha256"), r.Header.Get("Authorization"), r.ContentLength
 		b, _ := io.ReadAll(r.Body)
 		gotBody = string(b)
@@ -108,13 +108,13 @@ func TestListFollowsContinuationTokens(t *testing.T) {
 			return
 		}
 		if q.Get("continuation-token") == "" {
-			fmt.Fprint(w, `<ListBucketResult><IsTruncated>true</IsTruncated><NextContinuationToken>next</NextContinuationToken>
+			_, _ = fmt.Fprint(w, `<ListBucketResult><IsTruncated>true</IsTruncated><NextContinuationToken>next</NextContinuationToken>
 				<Contents><Key>database/minstrel-20261101T030000Z.tar</Key><Size>10</Size><LastModified>2026-11-01T03:00:05.000Z</LastModified></Contents>
 				<Contents><Key>database/minstrel-20261102T030000Z.tar</Key><Size>11</Size><LastModified>2026-11-02T03:00:05.000Z</LastModified></Contents>
 				</ListBucketResult>`)
 			return
 		}
-		fmt.Fprint(w, `<ListBucketResult><IsTruncated>false</IsTruncated>
+		_, _ = fmt.Fprint(w, `<ListBucketResult><IsTruncated>false</IsTruncated>
 			<Contents><Key>database/minstrel-20261103T030000Z.tar</Key><Size>12</Size><LastModified>2026-11-03T03:00:05.000Z</LastModified></Contents>
 			</ListBucketResult>`)
 	}))
@@ -131,9 +131,9 @@ func TestListFollowsContinuationTokens(t *testing.T) {
 }
 
 func TestARefusalSaysWhatIsWrong(t *testing.T) {
-	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		w.WriteHeader(http.StatusForbidden)
-		fmt.Fprint(w, `<Error><Code>SignatureDoesNotMatch</Code><Message>The request signature we calculated does not match</Message></Error>`)
+		_, _ = fmt.Fprint(w, `<Error><Code>SignatureDoesNotMatch</Code><Message>The request signature we calculated does not match</Message></Error>`)
 	}))
 	defer srv.Close()