refactor: unify env var helpers and remove magic slice in cache

- Add _getenv_num as shared core for _getenv_int/_getenv_float
- Add _getenv_optional_float for FRIGATE_SCORE_CEILING (replaces 9-line inline block)
- Add _getenv_bool; replace 11 inline .lower()-in-("true","1","yes") sites
  across config.py, jobs.py, and cli.py with single call site
- _resolve_strategy no-embedding branch: inline try/except → _getenv_int("LIMIT", 30)
- cache.py: final[:-4] → final.removesuffix(".npy") — assumption is now explicit

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "winnow"
-version = "0.5.15"
+version = "0.5.16"
 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"