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

Dockerfile

@@ -7,7 +7,7 @@ COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
 # Copy application
-COPY app.py models.py auth.py meals.py i18n.py ntfy.py ./
+COPY app.py models.py auth.py meals.py i18n.py ntfy.py send_reminders_cron.py ./
 COPY templates/ templates/
 COPY static/ static/