Refactor cron to call API via HTTP, capture base_url from browser

- meal-cron now runs send_reminders_cron.sh (simple wget loop, no DB mount)
- New /api/cron-tick endpoint handles all timezone logic server-side
- base_url captured from request.host_url when user saves ntfy settings,
  stored per-user — no APP_BASE_URL env var needed
- Cron endpoint protected by CRON_SECRET shared env var
- Removed send_reminders_cron.py, DB volume from cron container
- tests: 3 new cron-tick + base_url capture tests (69 total)

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 send_reminders_cron.py ./
+COPY app.py models.py auth.py meals.py i18n.py ntfy.py send_reminders_cron.sh ./
 COPY templates/ templates/
 COPY static/ static/