Detect and handle duplicate Immich people with the same name

When Immich has multiple person records sharing a name (e.g. unmerged
face clusters), winnow would previously run separate jobs for each,
with the second job wiping the first job's output directory — resulting
in far fewer training images than expected.

New behaviour:
- At startup, duplicate names are detected and a warning is printed
  showing asset counts for each duplicate.
- By default (MERGE_DUPLICATE_PEOPLE=false), only the person with the
  most assets is processed; smaller duplicates are skipped cleanly.
- With MERGE_DUPLICATE_PEOPLE=true, the duplicates are permanently
  merged inside Immich via PUT /api/people/{id}/merge (keeps the
  largest), then the people list is re-fetched before jobs run.

Also adds an explicit comment in executor.py confirming that replacement
targets come exclusively from tracker-mapped files, so manually-added
Frigate training images are never selected for deletion.

winnow/executor.py

@@ -376,6 +376,8 @@ def upload_to_frigate(jobs: list[dict]) -> None:
             # Snapshot live Frigate files for post-upload reconciliation diff only.
             # effective_count is sourced from the tracker (mapped files) so that
             # manually-added Frigate files don't consume winnow's managed quota.
+            # Replacement targets also come exclusively from the tracker, so manually
+            # added files are never selected for deletion — only winnow-uploaded ones.
             _snapshot = (
                 all_frigate_files.get(name, []) if all_frigate_files is not None
                 else get_frigate_person_files(name)