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_upload.py

@@ -596,6 +596,7 @@ DISPLAY_WRITE_SITES = {
     ("downloads/service.py", "enqueue_clip"): {"display_name or asset.title[:DISPLAY_TEXT_MAX]"},
     ("downloads/pipeline.py", "_download"): {"meta.title[:DISPLAY_TEXT_MAX]"},
     ("downloads/pipeline.py", "_process_clip"): {"named[:DISPLAY_TEXT_MAX]"},
+    ("downloads/backfill.py", "_retitle"): {"named[:DISPLAY_TEXT_MAX]"},
     ("worker.py", "_finish_from_cache"): {"asset.title[:DISPLAY_TEXT_MAX]"},
     ("downloads/upload.py", "ingest_upload"): {"display_text(display_name)"},
     ("routes/auth.py", "callback"): {"display_text(userinfo.get('name'))"},