fix: symlink guard placement, fetch_all_assets raw count, config TOCTOU (#31)

executor.py:
- Move islink check into _safe_person_dir on the raw path, before realpath
  resolves it; the previous check at the rmtree site was unreachable dead code
  because realpath already followed any symlink

immich_api.py / jobs.py:
- fetch_all_assets now returns (assets, total_raw) where total_raw is the
  item count seen before non-dict filtering; callers use it for MIN_FACE_COUNT
  guard and display so transient non-dict API items can't incorrectly skip people
- Add WARNING when pagination stops because a page had items but all were non-dict

config.py:
- Cache _data_cfg.exists() in _data_cfg_exists so the dual-config warning
  and config_file selection always read from the same stat() result; previously
  two calls created a TOCTOU window where log and code could disagree

Bump version to 0.5.7

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "winnow"
-version = "0.5.6"
+version = "0.5.7"
 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"