fix: resolve all golangci-lint errors and integrate linter into CI/CD, Makefile, and check script

internal/parser/parser.go

@@ -53,7 +53,9 @@ func (p *Parser) FetchFeed(ctx context.Context, feedURL, etag, lastModified stri
 	if err != nil {
 		return nil, fmt.Errorf("failed to fetch url: %w", err)
 	}
-	defer resp.Body.Close()
+	defer func() {
+		_ = resp.Body.Close()
+	}()
 
 	if resp.StatusCode == http.StatusNotModified {
 		return &FetchResult{NotModified: true}, nil