add headers.json and fix relative singlefile path resolving for sonic

archivebox/index/schema.py

@@ -427,6 +427,7 @@ class Link:
             'archive_org_path': 'https://web.archive.org/web/{}'.format(self.base_url),
             'git_path': 'git/',
             'media_path': 'media/',
+            'headers_path': 'headers.json',
         }
         if self.is_static:
             # static binary files like PDF and images are handled slightly differently.