fix: 4 correctness bugs from full-codebase audit

- executor: restore effective_count when replacement upload fails all retries
  (delete succeeded but slot was never filled, leaving cap undercount)
- diversity: skip zero-norm embeddings before dedup/FPS selection
  (InsightFace zeros pass dedup with similarity 0 and score distance 1.0,
  getting selected first as maximally diverse)
- cli: exclude None from skip_ids in _smaller_duplicate_ids
  (p.get('id') without None guard lets None into the set, silently
  dropping every other id-less person from the processed list)
- embeddings: select face nearest crop centre instead of largest by area
  (25% margin can pull a bigger neighbouring face into the crop;
  largest-face selection then embeds the wrong person)

winnow/executor.py

@@ -608,6 +608,12 @@ def upload_to_frigate(jobs: list[dict]) -> None:
                             progress.console.print(
                                 f"    [red]✗ {fname}: {type(e).__name__} - {e} (after {max_retries} attempts)[/red]"
                             )
+                    else:
+                        # All retries exhausted without a successful upload.
+                        # Restore the slot freed by the preceding delete so the next
+                        # candidate still sees at_cap=True and must beat the replacement gate.
+                        if at_cap:
+                            effective_count += 1
 
                     progress.advance(upload_task)