fix(logs): deep-review round 3 — a failure nobody could record is one scrubbed line

- The download and edit failure lines go back after their DB write: a job the
  write could not fail is not a failed job, and the worker's child_exit line
  already says it failed.
- The download child's runner is the last stop before Python's hook: an
  escaping exception is one download.child.failed line with its chain scrubbed,
  then exit 1, instead of a raw stderr traceback carrying the URL.
- A crash the database will not record carries its stack once, and the
  fail-write line has an event code.
- The job's own generic-extractor name is taken out as a literal (formats.hiding),
  so brackets and colons in a direct link's file name no longer leak; the
  pattern stays as the fallback. `Downloading just the video <id>` is covered.
- Tests: scrubbed_stack's cause, suppress, unprintable and text-format paths,
  the image steps' failure kinds, the watchdog error through the real
  _fail_running, exact poster/storyboard messages, ffmpeg skip marker.

backend/app/downloads/backfill.py

@@ -30,6 +30,11 @@ log = logs.get_logger(__name__)
 def _extract_meta(url: str, adapter: adapters.SourceAdapter, asset_id: int) -> dict | None:
     """Metadata-only yt-dlp extraction, using the source adapter's client sets + options (YouTube
     needs the POT/player-client strategy even just to extract)."""
+    with formats.hiding(url):
+        return _extract_meta_scoped(url, adapter, asset_id)
+
+
+def _extract_meta_scoped(url: str, adapter: adapters.SourceAdapter, asset_id: int) -> dict | None:
     for clients in adapter.client_sets():
         opts = formats.build_ydl_opts(
             {}, "%(id)s.%(ext)s", lambda d: None, extra=adapter.ydl_opts_extra(clients)