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).
peter
c5533b0ac5