refactor(api): shared serializer + ownership + write-scope helpers (R6 S1a)

Establish the R6 API-contract infra before the S1b payload:dict migration:
- iso(dt) in utils.py replaces the ~16 hand-rolled `x.isoformat() if x else None`
  serializer idioms across the routes.
- get_or_404 / owned_or_404 in routes/_common.py — one 404 + existence-leak policy;
  plex's _own_playlist_or_404 and saved_views' _own_view now delegate to it.
- require_write_scope dependency in auth.py collapses the inline has_write_scope→403
  checks (channels subscribe/unsubscribe, playlists push/push-plan) that had drifted
  into three different messages into one; the conditional delete_playlist guard stays
  inline (its message was already canonical).

No response-shape change (iso == isoformat for a present value). Gate: ruff clean,
pytest 163 green (+ test_utils for iso).

backend/app/routes/notifications.py

@@ -11,6 +11,7 @@ from sqlalchemy.orm import Session
 from app.auth import current_user
 from app.db import get_db
 from app.models import Notification, User
+from app.utils import iso
 from app.notifications import trim_read
 
 router = APIRouter(prefix="/api/me/notifications", tags=["notifications"])
@@ -25,7 +26,7 @@ def _serialize(n: Notification) -> dict:
         "data": n.data,
         "read": n.read,
         "dismissed": n.dismissed,
-        "created_at": n.created_at.isoformat() if n.created_at else None,
+        "created_at": iso(n.created_at),
     }