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/saved_views.py

@@ -6,6 +6,7 @@ from sqlalchemy.orm import Session
 from app.auth import require_human
 from app.db import get_db
 from app.models import SavedView, User
+from app.routes._common import owned_or_404
 
 # Saved "smart views": per-user named snapshots of the feed's FeedFilters. Gated behind
 # require_human so the shared demo account can't pollute a communal list (mirrors how other
@@ -71,10 +72,7 @@ def create_view(
 
 
 def _own_view(db: Session, user: User, view_id: int) -> SavedView:
-    view = db.get(SavedView, view_id)
-    if view is None or view.user_id != user.id:
-        raise HTTPException(status_code=404, detail="Unknown view")
-    return view
+    return owned_or_404(db, SavedView, view_id, user, detail="Unknown view")
 
 
 @router.patch("/{view_id}")