fix(downloads): Facebook files get their upload date, non-Latin names their letters

Since 2026-10-28 the signed-in Facebook read gives no upload date, so
every Facebook file since was named 0000-00-00 in Season_0 (31 on prod,
none before). The signed-out read that already titles these downloads
carries the date, so it now lends that too when the signed-in read has
none (backfill.public_title_info is now public_page_info).

On-disk names dropped every letter outside the Latin script, so a Hebrew,
Cyrillic, Greek or CJK uploader went to one shared "untitled" folder
(prod asset 503). anyascii now writes them in Latin letters
(Сергей Иванов -> Sergei_Ivanov); Latin and accented names are unchanged.

backend/requirements.txt

@@ -13,6 +13,10 @@ py3langid>=0.3,<1.0
 itsdangerous>=2.1,<3.0
 cryptography>=50.0.0,<51
 argon2-cffi>=23.1,<26
+# On-disk names are ASCII (storage._ascii_fold): a Hebrew, Cyrillic, Greek or CJK uploader name is
+# written in Latin letters instead of dropped whole into one shared "untitled" folder. Pure Python,
+# no dependencies of its own.
+anyascii>=0.3.3,<0.4
 # Download center: yt-dlp does the extraction/download; ffmpeg (OS-level in the Dockerfile)
 # does the merge + postprocessing. The [default] extra pulls yt-dlp-ejs (the challenge-solver
 # scripts Deno runs for YouTube's n-signature). yt-dlp is release-often (YouTube changes), so