Add cron container and per-household timezone for auto-reminders

- docker-compose.yml: meal-cron service loops send_reminders_cron.py every 60s
- send_reminders_cron.py: checks each household's local time, sends ntfy
  reminders at 10:00 (lunch) and 16:00 (dinner), guarded against duplicates
- models.py: timezone, last_lunch_reminder, last_dinner_reminder on households
- meals.py: /admin/timezone route for admins to set household timezone
- dashboard: timezone selector (20 common zones) in Account Settings
- i18n: 5 new translation keys for timezone strings
- Dockerfile: copy send_reminders_cron.py
- tests: 4 new timezone tests (admin set, non-admin denied, default UTC, auth)
- docs: updated AGENTS.md

AGENTS.md

@@ -22,8 +22,9 @@ Flask web app for tracking household meal attendance. Users register into househ
 | `static/style.css` | All CSS (no framework) |
 | `tests/` | Pytest suite (47 tests): `conftest.py` sets up in-memory DB per test |
 | `Dockerfile` | Python 3.13-alpine. **Must list every `.py` file in the COPY line.** |
-| `docker-compose.yml` | Single service `meal-tracker` on external `caddy` network, volume `meal-data` at `/data` |
+| `docker-compose.yml` | Two services: `meal-tracker` (web) and `meal-cron` (sends reminders). Both on external `caddy` network, shared `meal-data` volume at `/data`. |
 | `watch-for-updates.sh` | Polls git remote, rebuilds on change. Uses `docker compose` with env vars from `.env`. |
+| `send_reminders_cron.py` | Standalone script run every 60s by the `meal-cron` container. Checks per-household local time against 10:00/16:00 triggers. |
 | `doc/index.md` | Full project documentation |
 | `README.md` | Symlink → `doc/index.md` |