feat: make site prefix column sortable

Clicking the Prefix column header sorts device and rack lists by site
name, which starts with the prefix.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

netbox_utilities/tests/test_site_prefix.py

@@ -74,5 +74,9 @@ class SitePrefixTest(SimpleTestCase):
 
         self.assertIn(COLUMN_NAME, DeviceTable.base_columns)
         self.assertIn(COLUMN_NAME, RackTable.base_columns)
+        for table in (DeviceTable, RackTable):
+            column = table.base_columns[COLUMN_NAME]
+            self.assertTrue(column.orderable)
+            self.assertEqual(tuple(column.order_by), ("site__name",))
         self.assertEqual(DeviceTable.Meta.default_columns[:2], ("pk", COLUMN_NAME))
         self.assertEqual(RackTable.Meta.default_columns[:2], ("pk", COLUMN_NAME))