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/runner.py

@@ -21,6 +21,7 @@ from app import logsink
 from app.config import settings
 from app.db import SessionLocal
 from app.downloads import edit as editmod
+from app.downloads import formats
 from app.models import DownloadJob
 
 log = logs.get_logger(__name__)
@@ -81,14 +82,40 @@ def main() -> None:
     if asset_id is None:
         return
 
+    _run(job_id, asset_id, job_kind, source_kind, source_ref, spec, user_id)
+
+
+def _run(job_id, asset_id, job_kind, source_kind, source_ref, spec, user_id) -> None:
+    """Run the job, and be the last stop before Python's own exception hook.
+
+    What gets past the pipeline is a failure it could not even record (the database refused the
+    write that fails the job). Python's hook would print it to stderr raw — and its context is the
+    download's own failure, whose text is the URL or ffmpeg's stderr naming the file. Here it is
+    one line with the whole chain scrubbed; the non-zero exit then has the worker fail the job
+    (`cause=child_exit`)."""
     # Imported here (not at module top) so the child pays the yt-dlp import cost only when it has
     # real work, and to keep the import graph obvious: runner is a thin entry over the pipeline.
     from app.downloads.pipeline import _download, _process_edit
 
-    if job_kind == "edit":
-        _process_edit(job_id, asset_id)
-    else:
-        _download(job_id, asset_id, source_kind, source_ref, spec, user_id)
+    with formats.hiding(source_ref):
+        try:
+            if job_kind == "edit":
+                _process_edit(job_id, asset_id)
+            else:
+                _download(job_id, asset_id, source_kind, source_ref, spec, user_id)
+        except Exception as exc:  # noqa: BLE001 — whatever it is, it is reported here or raw
+            # noqa TRY400: the stack goes with it scrubbed (`scrubbed_stack`); `exc_info` is raw.
+            log.error(  # noqa: TRY400
+                "the download child for job %s failed",
+                job_id,
+                extra={
+                    **logs.event(
+                        "download.child.failed", job_id=job_id, error_type=type(exc).__name__
+                    ),
+                    **logs.scrubbed_stack(exc, formats.log_safe),
+                },
+            )
+            raise SystemExit(1) from None
 
 
 if __name__ == "__main__":