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)

send_reminders_cron.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+# Cron loop — calls the meal-tracker API every 60s to trigger reminders.
+# The Flask endpoint handles all timezone logic and ntfy sending.
+
+while true; do
+  wget -q -O- --post-data="" \
+    --header="X-Cron-Secret: ${CRON_SECRET}" \
+    http://meal-tracker:5000/api/cron-tick 2>/dev/null
+  sleep 60
+done