One hour-long appointment

[sedatcapar]

As a patient, I want to book an hour-long appointment, so that I can consult with a doctor.

Acceptance Criteria:

1. Appointment duration defaults to 60 minutes and checks slot availability.
2. Double booking is prevented with transactional lock or unique constraint.
3. Success page shows doctor, date, time, and cancellation policy.

Priority: High
Story Points: 5

Notes:

• Timezone handling on both UI and DB.