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/tests/test_download_errors.py

@@ -12,6 +12,7 @@ def test_real_ytdlp_messages_map_to_categories():
         "ERROR: [youtube] abc: Sign in to confirm your age. This video may be inappropriate": "login_required",
         "ERROR: [youtube] abc: Sign in to confirm you're not a bot": "login_required",
         "ERROR: [youtube] abc: Join this channel to get access to members-only content": "login_required",
+        "ERROR: [Reddit] 1vi1x56: Account authentication is required.": "login_required",
         "ERROR: [youtube] abc: The uploader has not made this video available in your country": "geo_blocked",
         "ERROR: [youtube] abc: Requested format is not available": "format_unavailable",
         "ERROR: [youtube] abc: Only images are available for download": "format_unavailable",