refactor(downloads): address /code-review on the download hotfix

- edit.py: extract _run_ffmpeg_best_effort (one run+timeout+swallow policy shared by
  thumbnail_to_jpg / ensure_poster / ensure_storyboard) + a build_thumbnail_jpg_cmd
  builder, matching the module's other ffmpeg commands.
- thumbnail_to_jpg: copy an already-JPEG thumbnail (Facebook, and Reddit's jpeg-under-
  a-.png-name) instead of re-encoding it; only webp/png-proper are decoded via ffmpeg.
- errors.py: drop the redundant "account authentication" classify marker
  ("authentication is required" already covers the known Reddit message).
- formats.py: document that the opt-in EmbedThumbnail is unaffected by dropping the
  convertor (it converts unsupported thumbs to PNG, not the crashing mjpeg2jpeg path).
- test_titles.py: guard the ReDoS regression with a thread + timeout (deterministic
  "must not hang") instead of a wall-clock assertion.

backend/app/downloads/errors.py

@@ -24,7 +24,6 @@ _RULES: tuple[tuple[str, str], ...] = (
     # Some extractors (e.g. Reddit) gate content behind a logged-in account: "Account
     # authentication is required." — an auth wall we deliberately don't cross, not a transient error.
     ("authentication is required", "login_required"),
-    ("account authentication", "login_required"),
     # Geo-restricted.
     ("in your country", "geo_blocked"),
     ("available in your location", "geo_blocked"),