fix: strip empty env vars in _getenv_num/_getenv_bool; unify FORCE_CPU

- _getenv_num: add raw.strip() + empty-string guard so numeric vars set
  to "" (common Compose pattern for "use default") return the default
  silently instead of warning "not a valid int/float"
- _getenv_bool: same guard so True-defaulted flags set to "" return
  the configured default instead of silently returning False
- embeddings.py: replace inline FORCE_CPU bool parse with _getenv_bool

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "winnow"
-version = "0.5.16"
+version = "0.5.17"
 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"