fix: address 10 full-codebase audit findings (v0.5.19)

Correctness:
- fetch_face_data: only fall back to faces[0] when person_id is absent;
  previously a missing person match injected a different person's bbox
- upload_tracker: change PK from (asset_id, status) to
  (asset_id, person_name, status); old PK allowed INSERT OR REPLACE to
  silently overwrite person_name when the same photo appeared in two
  people's jobs, breaking quality-replacement JOINs; auto-migrates DBs
- filter_recent_assets: treat years=0 as "no age filter" instead of
  falling through to Config.YEARS_FILTER via falsy `or`
- _is_module_available: return find_spec(...) is not None; find_spec
  returns None (not raises) for absent top-level modules, so the
  previous code always returned True
- execute_jobs error handler: use asset.get("id", "<unknown>") to avoid
  a secondary KeyError propagating out of execute_jobs on malformed dicts
- upload_to_frigate: also mark_rejected on HTTP 422, not only HTTP 400
  with "face" in body; other permanent errors left assets untracked and
  retried forever
- reconcile_frigate_mappings: sort key lambda f: (_ts(f), f) makes order
  deterministic when timestamps are equal or 0.0; set iteration order is
  hash-randomised, stable sort preserves it

Reuse / cleanup:
- config.py: add _getenv_optional_int delegating to _getenv_num(name, None, int)
- jobs.py: _resolve_strategy uses _getenv_optional_int("LIMIT") instead
  of inline os.environ.get + int() + warning duplicate of _getenv_num
- frigate_api.py: add _get_frigate_url() helper; eliminates 4× copy of
  os.environ.get("FRIGATE_URL", "").rstrip("/")
- quality.py: extract blur_score_from_image(img, max_dim=1440) helper;
  executor.py time-spread blur fallback now uses it instead of inlining
  the resize+RGB+assess_quality sequence, keeping scale logic in one place

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "winnow"
-version = "0.5.18"
+version = "0.5.19"
 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"