fix(downloads): a Facebook download titled only "Facebook" takes the post's text

Since 2026-10-28 a signed-in read of a reel gives the placeholder title and
no description, so the adapter had nothing to fall back on and every such
download (and its name) read "Facebook". A placeholder with nothing behind
it now gets one signed-out, metadata-only read of the page. The backfill
retitles the downloads already made, and every name that only repeated the
placeholder; a name its owner gave stays.

backend/tests/test_failure_lines.py

@@ -368,7 +368,7 @@ def test_the_metadata_backfill_names_the_asset_and_not_the_download(sink, monkey
             )
 
     monkeypatch.setattr(backfill.yt_dlp, "YoutubeDL", _Refusing)
-    assert backfill._extract_meta(url, adapters.YouTubeAdapter(), 11) is None
+    assert backfill.extract_meta(url, adapters.YouTubeAdapter(), 11) is None
 
     lines = [o for o in sink.objects() if "meta extract failed" in o["msg"]]
     assert lines and all("dQw4w9WgXcQ" not in json.dumps(o) for o in lines)
@@ -393,7 +393,7 @@ def test_the_metadata_backfill_takes_out_a_generic_name_whatever_it_holds(sink,
             raise yt_dlp.utils.DownloadError("ERROR: [generic] Our Trip: Day 1: Unable to download")
 
     monkeypatch.setattr(backfill.yt_dlp, "YoutubeDL", _Refusing)
-    assert backfill._extract_meta(url, adapters.SourceAdapter(), 11) is None
+    assert backfill.extract_meta(url, adapters.SourceAdapter(), 11) is None
 
     lines = [o for o in sink.objects() if "meta extract failed" in o["msg"]]
     assert lines and all("Day 1" not in json.dumps(o) for o in lines)