fix: treat Frigate 500 'Could not process' as permanent rejection

Frigate returns HTTP 500 with 'Could not process' when its face detector
cannot find or embed a face in the uploaded crop — this will never succeed
on retry. Previously these were silently logged at DEBUG and retried on
every future run.

- Surface 500 error details inline (same display path as 400)
- Mark 500 + 'could not process' as a permanent rejection so the asset
  is skipped on future runs instead of retried indefinitely

winnow/executor.py

@@ -566,13 +566,14 @@ def upload_to_frigate(jobs: list[dict]) -> None:
                                     error_detail = resp.json().get("message", full_body[:100])
                                 except Exception:
                                     error_detail = full_body[:100]
-                                if resp.status_code == 400:
+                                if resp.status_code in (400, 500):
                                     progress.console.print(f"      [dim]{error_detail}[/dim]")
                                 else:
                                     logger.debug("%s HTTP %s: %s", fname, resp.status_code, error_detail)
                                 _is_permanent = (
                                     (resp.status_code == 400 and "face" in full_body.lower())
                                     or resp.status_code == 422
+                                    or (resp.status_code == 500 and "could not process" in full_body.lower())
                                 )
                                 if _is_permanent:
                                     asset_id = asset_map.get(fname)