feat: safe backdated meal logging

[maxjustships]

Summary

• add strict local-date --date support for parsed and direct meal logs at deterministic local noon
• return logged_at and local_date, reject malformed/impossible/future dates without writes, and add stats date
• cover controlled-timezone boundaries, update CLI/agent guidance, and keep existing data/schema compatible

Verification

• env -u NOMNOM_OFFLINE TZ=Asia/Almaty PYTHONPATH=. pytest -q (189 passed)
• ruff check .
• git diff --check
• disposable installed-CLI smoke for parsed/direct 2026-07-20 logs plus date-scoped stats

Closes #27