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_thumbnail_poster.py

@@ -0,0 +1,46 @@
+"""`thumbnail_to_jpg`: robustly turn a downloaded thumbnail into a real JPEG poster/og:image.
+
+This replaced yt-dlp's `FFmpegThumbnailsConvertor`, whose `-bsf:v mjpeg2jpeg` path crashed on some
+already-JPEG thumbnails (Reddit serves a JPEG under a `.png` name) — and because a postprocessor
+error is fatal, it used to fail the WHOLE download. The key regression property here is that a bad
+thumbnail returns None (poster falls back to a cut frame) instead of raising.
+"""
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+from app.downloads import edit
+
+_ffmpeg_missing = shutil.which("ffmpeg") is None
+needs_ffmpeg = pytest.mark.skipif(_ffmpeg_missing, reason="ffmpeg not available")
+
+
+def _is_jpeg(p: Path) -> bool:
+    return p.exists() and p.read_bytes()[:3] == b"\xff\xd8\xff"
+
+
+@needs_ffmpeg
+def test_converts_a_png_to_a_real_jpeg(tmp_path):
+    src = tmp_path / "thumb.png"
+    subprocess.run(
+        ["ffmpeg", "-y", "-loglevel", "error", "-f", "lavfi",
+         "-i", "color=c=red:s=64x64", "-frames:v", "1", str(src)],
+        check=True,
+    )
+    out = edit.thumbnail_to_jpg(src, tmp_path / "poster.jpg")
+    assert out is not None and _is_jpeg(out)
+
+
+def test_returns_none_for_a_missing_source(tmp_path):
+    assert edit.thumbnail_to_jpg(None, tmp_path / "p.jpg") is None
+    assert edit.thumbnail_to_jpg(tmp_path / "nope.png", tmp_path / "p.jpg") is None
+
+
+@needs_ffmpeg
+def test_returns_none_on_garbage_input_instead_of_raising(tmp_path):
+    # The whole point: a thumbnail ffmpeg can't decode must NOT crash the download.
+    bad = tmp_path / "bad.png"
+    bad.write_bytes(b"this is definitely not an image")
+    assert edit.thumbnail_to_jpg(bad, tmp_path / "p.jpg") is None