Mark Unavailability

# User Story

**Title:**
_As a doctor, I want to mark my unavailable dates and times, so that patients can only book available slots._

**Acceptance Criteria:**
1. Doctor can mark unavailable dates.
2. Unavailable slots cannot be booked.
3. Changes are reflected immediately.

**Priority:** High

**Story Points:** 5

**Notes:**
- Existing appointments should not be affected.