fix(logs): deep-review round 2 — a stack names code, not data, and survives a failed write

- A failure keeps its traceback unless it is yt-dlp's or our own StatedFailure (a
  message that is the whole story). A plain RuntimeError, RecursionError or OSError
  is also what a bug of ours raises, and its stack is the only lead.
- The stack is scrubbed link by link (logs.scrubbed_stack): a chained DownloadError
  under a DB error no longer prints yt-dlp's text unscrubbed.
- Engine hide_parameters: a DB error's message no longer carries the title or path
  it was writing.
- The download and edit failure lines go out even when their DB write fails.
- The worker's child_exit/crash/supervisor lines say download.job.failed only when
  the job failed there; a crash the DB will not record no longer ends the thread.
  Stopping a paused child is download.child.stopped, not a second abort.
- log_safe also covers generic ids with spaces, playlist titles and bare ids.
- The ffmpeg image steps log the failure kind, not the argv with its path.
- The consent contract accepts only the body of a plain consented_trace() check,
  and any level that may be TRACE.
- Tests for each; every one mutation-checked.

backend/app/downloads/edit.py

@@ -34,6 +34,7 @@ from pathlib import Path
 
 from app import logs
 from app.config import settings
+from app.downloads.stated import StatedFailure
 
 log = logs.get_logger(__name__)
 
@@ -451,11 +452,11 @@ def run_ffmpeg(
             if should_cancel():
                 raise EditAborted
             if time.monotonic() - last_output > stall_timeout_s:
-                raise RuntimeError(f"ffmpeg stalled: no progress for {int(stall_timeout_s)}s")
+                raise StatedFailure(f"ffmpeg stalled: no progress for {int(stall_timeout_s)}s")
         try:
             ret = proc.wait(timeout=_FFMPEG_EXIT_GRACE_S)
         except subprocess.TimeoutExpired:  # pipes closed but the process lingers
-            raise RuntimeError("ffmpeg did not exit after closing its output")
+            raise StatedFailure("ffmpeg did not exit after closing its output")
     except BaseException:
         # ONE exit path stops the process, so the invariant is unconditional: nothing leaves this
         # function with ffmpeg still running. Not just our own EditAborted/stall — `on_progress`
@@ -470,7 +471,7 @@ def run_ffmpeg(
             if pipe:
                 pipe.close()
     if ret != 0:
-        raise RuntimeError(f"ffmpeg failed ({ret}): {''.join(err_tail)[-300:]}")
+        raise StatedFailure(f"ffmpeg failed ({ret}): {''.join(err_tail)[-300:]}")
 
 
 # --- ffmpeg: universal browser playability --------------------------------------------------
@@ -723,6 +724,15 @@ def _run_ffmpeg_best_effort(cmd: list[str], timeout_s: int) -> Exception | None:
         return exc
 
 
+def _step_failure(err: BaseException) -> str:
+    """What a best-effort image step's failure line says: the kind and, for ffmpeg, its exit
+    status. Never `str(err)`: a CalledProcessError or TimeoutExpired quotes ffmpeg's argv, and an
+    OSError its file name — the source path either way, which is built from the channel and the
+    title."""
+    code = getattr(err, "returncode", None)
+    return f"{type(err).__name__} {code}" if code is not None else type(err).__name__
+
+
 def _is_jpeg(path: Path) -> bool:
     """True if the file starts with the JPEG SOI marker (FF D8 FF)."""
     try:
@@ -750,13 +760,15 @@ def thumbnail_to_jpg(src: Path | None, dest: Path, timeout_s: int = 30) -> Path
             shutil.copyfile(src, dest)
             return dest
         except OSError as exc:
-            log.info("thumbnail copy failed (%s); poster falls back to a cut frame", str(exc)[:120])
+            log.info(
+                "thumbnail copy failed (%s); poster falls back to a cut frame", _step_failure(exc)
+            )
             return None
     err = _run_ffmpeg_best_effort(build_thumbnail_jpg_cmd(src, dest), timeout_s)
     if err is not None:
         log.info(
             "thumbnail->jpg conversion failed (%s); poster falls back to a cut frame",
-            str(err)[:120],
+            _step_failure(err),
         )
         return None
     return dest if dest.exists() else None
@@ -781,7 +793,7 @@ def ensure_poster(asset, download_root: str, timeout_s: int = 30) -> str | None:
     at = min(3.0, (asset.duration_s or 10) * 0.1)
     err = _run_ffmpeg_best_effort(build_poster_cmd(src, dest, at), timeout_s)
     if err is not None:
-        log.info("poster gen skipped for asset %s: %s", asset.id, str(err)[:120])
+        log.info("poster gen skipped for asset %s: %s", asset.id, _step_failure(err))
         return None
     return rel if dest.exists() else None
 
@@ -812,7 +824,7 @@ def ensure_storyboard(asset, download_root: str, timeout_s: int = 90) -> dict |
     cmd = build_storyboard_cmd(src, dest, asset.duration_s, geom)
     err = _run_ffmpeg_best_effort(cmd, timeout_s)
     if err is not None:
-        log.info("storyboard gen skipped for asset %s: %s", asset.id, str(err)[:120])
+        log.info("storyboard gen skipped for asset %s: %s", asset.id, _step_failure(err))
         return None
     if not dest.exists():
         return None