perf(notifications): bulk-delete trimmed read notifications (C-4.1)

trim_read looped a db.get()+db.delete() per id (N+1). The ids are already in hand, so
one DELETE ... WHERE id IN (...) replaces the loop.

backend/app/notifications.py

@@ -5,7 +5,7 @@ survive reloads/devices and are produced server-side. First producer is the main
 job. Unread rows never auto-expire; read rows are trimmed past a soft per-user cap so the
 table can't grow without bound for a heavy user.
 """
-from sqlalchemy import select
+from sqlalchemy import delete, select
 from sqlalchemy.orm import Session
 
 from app.models import Notification
@@ -52,7 +52,7 @@ def trim_read(db: Session, user_id: int, cap: int = READ_SOFT_CAP) -> int:
     )
     if not ids:
         return 0
-    for nid in ids:
+    # One bulk DELETE rather than a get+delete per row (the ids are already in hand).
-        db.delete(db.get(Notification, nid))
+    db.execute(delete(Notification).where(Notification.id.in_(ids)))
     db.commit()
     return len(ids)