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/tests/test_titles.py

@@ -1,4 +1,9 @@
 """normalize_title: de-shout / de-clutter video titles without mangling non-English text."""
+from concurrent.futures import ThreadPoolExecutor
+from concurrent.futures import TimeoutError as FuturesTimeout
+
+import pytest
+
 from app.titles import normalize_title
 
 
@@ -29,20 +34,26 @@ def test_numeric_episode_marker_is_not_treated_as_a_hashtag():
     assert normalize_title("My show #3") == "My show #3"
 
 
-def test_midstring_hashtags_are_fast_and_kept_redos_regression():
+def test_midstring_hashtags_dont_hang_and_are_kept_redos_regression():
     # Facebook prepends an engagement prefix and appends a page name, leaving a long hashtag cluster
     # in the MIDDLE. That is not a trailing SEO cluster, so the tags stay — and, critically, this
     # exact shape used to trigger catastrophic backtracking (ReDoS) in `_TRAILING_HASHTAGS` that
     # pinned a download-worker thread at 100% CPU forever inside the yt-dlp progress hook.
-    import time
-
     raw = (
         "398K views · 6.1K reactions | The money should follow 🙄 #reel #reels #meme #memes "
         "#fun #funny #work #worklife #office #viral #trending #fyp #foryou #foryoupage | Memebonanza4U"
     )
-    t = time.perf_counter()
-    out = normalize_title(raw) or ""
-    assert time.perf_counter() - t < 1.0  # linear now; the old regex ran for minutes / never returned
+    # Run in a worker thread with a generous ceiling: the fixed regex returns in microseconds, so
+    # only a genuine catastrophic-backtracking regression fails to complete — a hang is caught
+    # deterministically rather than by asserting on wall-clock slack in the happy path.
+    ex = ThreadPoolExecutor(max_workers=1)
+    fut = ex.submit(normalize_title, raw)
+    try:
+        out = fut.result(timeout=5) or ""
+    except FuturesTimeout:
+        pytest.fail("normalize_title did not return within 5s — catastrophic backtracking regressed")
+    finally:
+        ex.shutdown(wait=False)  # never join a (hypothetically) wedged thread
     assert "#foryoupage" in out and "#reel" in out  # mid-string hashtags preserved (not trailing)
     assert "views" not in out  # the engagement-count prefix was still stripped