Implement appointment reminder notification service

Create a module that reads upcoming appointments from the database and sends email reminders to patients.

Function:

sendAppointmentReminders(daysAhead: number): Promise<ReminderResult>
// ReminderResult: { sent: number, failed: number, skipped: number, errors: string[] }

Requirements:

1. Query appointments where appointment_date is within the next daysAhead days AND reminder_sent_at IS NULL
2. Send reminder emails via Resend (RESEND_API_KEY env var). If not set, log to console instead (dev fallback)
3. Email subject: Reminder: Your appointment on [date] at [time]
4. After successful send, set reminder_sent_at = now() on the appointment row
5. Retry failed sends up to 3 times with 2s exponential backoff
6. Idempotency: a second call within the same hour should skip already-reminded appointments
7. Return a summary of sent, failed, and skipped counts

Write integration tests using a real test database (not mocks). Cover: normal send, missing email address, already-reminded guard.