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/formats.py

@@ -218,6 +218,9 @@ def build_ydl_opts(
     # The `<name>.jpg` poster sidecar is instead produced robustly by our own pipeline (worker
     # `_download` → `editmod.thumbnail_to_jpg`, with `ensure_poster` cutting a video frame as a
     # fallback), so the og:image share preview (Facebook Messenger / MS Teams) keeps working.
+    # The opt-in EmbedThumbnail below is unaffected: it does its OWN format handling — it embeds a
+    # jpg/png thumbnail directly and converts anything else to PNG (not the crashing mjpeg2jpeg-to-jpg
+    # path), so it never needed the convertor and does not reintroduce the crash.
     if s["embed_thumbnail"]:
         opts["postprocessors"].append(
             {"key": "EmbedThumbnail", "already_have_thumbnail": True}