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/invites.py

@@ -15,6 +15,7 @@ from invite_utils import (
     normalize_identifier,
 )
 from models import Invite, InviteStatus, Permission, User
+from pagination import calculate_offset, create_paginated_response
 from schemas import (
     AdminUserResponse,
     InviteCheckResponse,
@@ -188,10 +189,9 @@ async def list_all_invites(
     # Get total count
     count_result = await db.execute(count_query)
     total = count_result.scalar() or 0
-    total_pages = (total + per_page - 1) // per_page if total > 0 else 1
 
     # Get paginated invites (relationships loaded via selectin)
-    offset = (page - 1) * per_page
+    offset = calculate_offset(page, per_page)
     query = query.order_by(desc(Invite.created_at)).offset(offset).limit(per_page)
     result = await db.execute(query)
     invites = result.scalars().all()
@@ -199,13 +199,7 @@ async def list_all_invites(
     # Build responses using preloaded relationships
     records = [build_invite_response(invite) for invite in invites]
 
-    return PaginatedInviteRecords(
-        records=records,
-        total=total,
-        page=page,
-        per_page=per_page,
-        total_pages=total_pages,
-    )
+    return create_paginated_response(records, total, page, per_page)
 
 
 @admin_router.post("/invites/{invite_id}/revoke", response_model=InviteResponse)