fix: v0.5.21 — diversity cap, fetch rejection, tracker isolation, merge dedup, env parsing

- diversity: cap k-medoids seed count at target so _cluster_aware_selection
  never returns more images than requested (violated MAX_AUTO_IMAGES when
  remaining capacity was 1-4 slots); add early return for limit=0 to
  prevent k-medoids from running with a zero budget; slice return to target
  as a final guard
- executor: mark_rejected() when fetch_full_image returns None so assets
  that can't be fetched (both original and preview) aren't retried every run
- executor: wrap mark_uploaded() in its own try/except so a SQLite disk-full
  error after a successful HTTP 200 doesn't retry the Frigate POST (duplicate
  upload) — the upload succeeded; only the tracker write failed
- cli: apply skip_ids deduplication to the re-fetched people list after a
  partial merge (some groups succeed, some fail) so unmerged duplicates
  don't produce two jobs for the same Frigate folder
- jobs: strip whitespace from SKIP_PEOPLE/ONLY_PEOPLE elements on split
  so "Alice, Bob" (space after comma) correctly matches "Bob"

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "winnow"
-version = "0.5.20"
+version = "0.5.21"
 description = "Selects diverse, high-quality photos from Immich as training data for Frigate face recognition."
 license = "AGPL-3.0-or-later"
 requires-python = ">=3.13"