Postgres + pg_cron scheduled shift reminders

1) Run migrations
- Apply `supabase/migrations/20260318_add_scheduled_notifications.sql` (and other pending migrations) to your Supabase DB.

2) Enable pg_cron (requires Supabase project admin)
- If your Supabase tier supports it, enable the `pg_cron` extension.
- Example (run as a privileged role):
  CREATE EXTENSION IF NOT EXISTS pg_cron;
  SELECT cron.schedule('shift_reminders_every_min', '*/1 * * * *', $$SELECT public.enqueue_due_shift_notifications();$$);

3) Deploy processor Edge Function
- Add `supabase/functions/process_scheduled_notifications/` to your Supabase functions and deploy with the following required env vars:
  - `SUPABASE_URL`
  - `SUPABASE_SERVICE_ROLE_KEY`
  - `SEND_FCM_URL` (the HTTP URL for your existing `send_fcm` function, e.g., https://<project>.functions.supabase.co/send_fcm)
  - Optional: `PROCESSOR_BATCH_SIZE` (default 50)

- You can deploy via `supabase functions deploy process_scheduled_notifications` or using your CI.

4) Scheduling / triggering processor
- Option A (recommended): Use `pg_cron` to only enqueue rows, and configure a small interval GitHub Actions or Cloud Scheduler to call the Edge Function endpoint (POST) every minute. This keeps sending out of the DB.
- Option B: Use `pg_cron` to call the Edge Function HTTP endpoint directly if `pg_http` is available (not recommended unless approved).

5) Verification
- Insert a test `duty_schedules` record 15 minutes ahead and run `SELECT public.enqueue_due_shift_notifications();` manually — confirm `scheduled_notifications` row appears.
- Call the Edge Function (`supabase functions invoke process_scheduled_notifications --project <id>`) or POST to its URL — confirm `send_fcm` receives payload and `scheduled_notifications` row becomes processed.

6) Monitoring
- Watch `cron.job_run_details` for cron runs and `scheduled_notifications` rows with `retry_count` > 3.
- Inspect `notification_pushes` table to ensure deduplication works.