Fix misleading HTTP 500 detail and RuntimeWarning on single-image selection

- HTTP 500 errors from Frigate no longer echo the response body to the user
  (Frigate's generic message says 'Try restarting Frigate' which is wrong —
  500s on upload are almost always image-specific, not a health issue). The
  detail is now logged at debug level. HTTP 400 detail is still shown since
  'No face was detected' is genuinely useful.
- np.median on empty upper triangle (n=1 after quality filtering) no longer
  emits RuntimeWarning; _compute_adaptive_threshold returns the floor (0.05)
  immediately when there are no pairwise distances to sample.
- k-medoids cluster count floor raised to 1 (was 0 when n < 3), preventing
  k=0 being passed to _kmedoids.

winnow/executor.py

@@ -580,10 +580,12 @@ def upload_to_frigate(jobs: list[dict]) -> None:
                             )
                             try:
                                 error_detail = resp.json().get("message", resp.text[:100])
-                                progress.console.print(f"      [dim]{error_detail}[/dim]")
                             except Exception:
                                 error_detail = resp.text[:100]
+                            if resp.status_code == 400:
                                 progress.console.print(f"      [dim]{error_detail}[/dim]")
+                            else:
+                                logger.debug(f"{fname} HTTP {resp.status_code}: {error_detail}")
                             if resp.status_code == 400 and "face" in error_detail.lower():
                                 asset_id = asset_map.get(fname)
                                 if asset_id: