test(backup): pin BACKUP_VERSION 15 (#4052)

The subscription and suppression sections left the payload in 0bcd4b5; the
version moved with them and the pin did not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/test_services_backup.py

@@ -23,7 +23,7 @@ def test_backup_version_is_current():
 
     (Named for the number it asserted until v10, which is exactly the drift a
     name-carrying-a-value invites; it now says what it checks.)"""
-    assert backup.BACKUP_VERSION == 14
+    assert backup.BACKUP_VERSION == 15
 
 
 def _exportable_note(**over):