refactor(backend): extract pagination utilities

Issue #4: Pagination logic was repeated across multiple routes.

Changes:
- Add pagination.py with reusable utilities:
  - calculate_total_pages: computes page count from total/per_page
  - calculate_offset: computes offset for given page
  - create_paginated_response: builds PaginatedResponse with metadata
- Update routes/audit.py to use pagination utilities
- Update routes/booking.py to use pagination utilities
- Update routes/invites.py to use pagination utilities

The utilities handle the common pagination math while routes
still manage their own query logic (filters, joins, ordering).

backend/routes/booking.py

@@ -12,6 +12,7 @@ from sqlalchemy.orm import joinedload
 from auth import require_permission
 from database import get_db
 from models import Appointment, AppointmentStatus, Availability, Permission, User
+from pagination import calculate_offset, create_paginated_response
 from schemas import (
     AppointmentResponse,
     AvailableSlotsResponse,
@@ -325,10 +326,9 @@ async def get_all_appointments(
     # Get total count
     count_result = await db.execute(select(func.count(Appointment.id)))
     total = count_result.scalar() or 0
-    total_pages = (total + per_page - 1) // per_page if total > 0 else 1
 
     # Get paginated appointments with explicit eager loading of user relationship
-    offset = (page - 1) * per_page
+    offset = calculate_offset(page, per_page)
     result = await db.execute(
         select(Appointment)
         .options(joinedload(Appointment.user))
@@ -344,13 +344,7 @@ async def get_all_appointments(
         for apt in appointments
     ]
 
-    return PaginatedAppointments(
-        records=records,
-        total=total,
-        page=page,
-        per_page=per_page,
-        total_pages=total_pages,
-    )
+    return create_paginated_response(records, total, page, per_page)
 
 
 @admin_appointments_router.post(