fix(downloads): stop a ReDoS title and a fatal thumbnail convert from wedging/killing downloads

Three prod-observed download bugs, root-caused from a frozen prod queue with the
worker pinned at 100% CPU:

- ReDoS in normalize_title: _TRAILING_HASHTAGS had two overlapping unbounded
  quantifiers around the required letter, so a Facebook title with a long hashtag
  cluster that is NOT at the end ("... #a #b #c | Page") backtracked catastrophically.
  It runs inside the yt-dlp progress hook holding the GIL, so one such download froze
  the whole worker (and could stall feed enrichment too - sync/videos.py calls it).
  Rewrite with a zero-width "has a letter" lookahead + a possessive tag body: one
  unambiguous split, nothing to backtrack. 40-tag stress case: ~0.1ms.

- Reddit download killed by a cosmetic thumbnail convert: yt-dlp's
  FFmpegThumbnailsConvertor runs "-bsf:v mjpeg2jpeg" on a JPEG-served-under-a-.png-name
  (Reddit) and fails; being a postprocessor, it failed the WHOLE download even though
  the media was fine. Drop it and produce the <base>.jpg poster/og:image ourselves via
  editmod.thumbnail_to_jpg (plain ffmpeg decode->mjpeg, best-effort -> ensure_poster
  cuts a frame on failure). The FB Messenger / MS Teams share preview keeps working
  (posters verified as real JPEGs).

- classify: map "authentication is required" (Reddit's auth wall) to login_required
  instead of unknown, so the UI explains it rather than offering a pointless retry.

Tests: ReDoS-timing + numeric-marker regression, the Reddit classify case, and
thumbnail_to_jpg (real JPEG out / None on bad input, never raises).

backend/app/worker.py

@@ -617,8 +617,12 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
         layout, retention_days = _download_settings()
         rel = storage.rel_path(meta, ext, layout, asset_id=asset_id)
         thumb = _find_thumbnail(staging, produced)
+        # Convert the raw thumbnail (webp/png/jpeg) to a real JPEG ourselves — robustly — for the
+        # poster + og:image sidecar, now that yt-dlp's fatal thumbnail convertor is gone. On failure
+        # this is None and write_sidecars writes no poster, so ensure_poster cuts a frame instead.
+        poster_jpg = editmod.thumbnail_to_jpg(thumb, staging / "poster.jpg")
         storage.place_file(produced, settings.download_root, rel)
-        nfo_ok = storage.write_sidecars(settings.download_root, rel, meta, thumb)
+        nfo_ok = storage.write_sidecars(settings.download_root, rel, meta, poster_jpg)
 
         final = storage.abs_path(settings.download_root, rel)
         size = final.stat().st_size if final.exists() else None