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/pagination.py

@@ -0,0 +1,46 @@
+"""Pagination utilities for API responses."""
+
+from typing import TypeVar
+
+from pydantic import BaseModel
+
+from schemas import PaginatedResponse
+
+RecordT = TypeVar("RecordT", bound=BaseModel)
+
+
+def calculate_total_pages(total: int, per_page: int) -> int:
+    """Calculate total number of pages."""
+    return (total + per_page - 1) // per_page if total > 0 else 1
+
+
+def calculate_offset(page: int, per_page: int) -> int:
+    """Calculate the offset for a given page."""
+    return (page - 1) * per_page
+
+
+def create_paginated_response(
+    records: list[RecordT],
+    total: int,
+    page: int,
+    per_page: int,
+) -> PaginatedResponse[RecordT]:
+    """
+    Create a paginated response with calculated metadata.
+
+    Args:
+        records: List of records for the current page
+        total: Total count of all records (before pagination)
+        page: Current page number (1-indexed)
+        per_page: Number of records per page
+
+    Returns:
+        A PaginatedResponse with all pagination metadata
+    """
+    return PaginatedResponse[RecordT](
+        records=records,
+        total=total,
+        page=page,
+        per_page=per_page,
+        total_pages=calculate_total_pages(total, per_page),
+    )