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/notifications.py

@@ -9,6 +9,7 @@ table can't grow without bound for a heavy user.
 from sqlalchemy import delete, select
 from sqlalchemy.orm import Session
 
+from app.userscope import owned_by
 from app.models import Notification
 
 # Keep at most this many READ notifications per user; older read ones are pruned. Unread
@@ -52,7 +53,7 @@ def trim_read(db: Session, user_id: int, cap: int = READ_SOFT_CAP) -> int:
     ids = (
         db.execute(
             select(Notification.id)
-            .where(Notification.user_id == user_id, Notification.read.is_(True))
+            .where(owned_by(Notification.user_id, user_id), Notification.read.is_(True))
             .order_by(Notification.created_at.desc())
             .offset(cap)
         )