refactor(core): the core's owner filters go through userscope, each held by a second account

Tokens, notifications, quota, debug sessions and the reset-link burn filtered
their owner inline. Ten of the fifteen filters had no test that noticed one
going missing; each now has a test where another account's rows must not
count, show or change.

backend/app/auth.py

@@ -4,6 +4,7 @@ from sqlalchemy import delete, func, select
 from sqlalchemy.orm import Session
 from starlette.requests import HTTPConnection
 
+from app.userscope import owned_by
 from app import debugsession
 from app import email as email_mod
 from app import logs
@@ -230,7 +231,7 @@ def purge_user(db: Session, user: User, background: BackgroundTasks) -> None:
     # request's access line, a queued row, a later background task would otherwise put rows back
     # under the id after this DELETE (logs.forget_user).
     logs.forget_user(user_id)
-    db.execute(delete(LogEvent).where(LogEvent.user_id == user_id))
+    db.execute(delete(LogEvent).where(owned_by(LogEvent.user_id, user_id)))
     # Every canonically-equal invite goes, not just the literal spelling: is_allowed matches on the
     # canonical form, so leaving a sibling row behind would keep granting access to a mailbox whose
     # account was just erased — and prod really does carry approved pairs for one mailbox.