refactor(api): code-review fixes — widen iso() to date, fix import order (R6 S1a)

/code-review high round 1 (5 findings):
- iso(dt) now typed `date | None` — MediaAsset.upload_date is a plain `date`, not a
  datetime; the helper's signature was too narrow for the type-contract epic. +test.
- restore alphabetical app.* import order in audit.py and plex.py (the new iso/_common
  imports had been dropped in mid-block).
The other 3 findings are deliberate design choices, left as-is for UAT: push/push-plan
now gate write-scope before the 404 (dependency ordering); the unified 403 message; and
owned_or_404's owner_attr kept for S1b's DownloadShare.to_user_id-style owners.

Gate: ruff clean, pytest 164 green.

backend/app/routes/plex.py

@@ -41,16 +41,16 @@ from app.models import (
     PlexState,
     User,
 )
-from app.routes._common import owned_or_404
-from app.utils import iso
 from app.plex import paths as plex_paths
 from app.plex import people as plex_people
 from app.plex import stream as plex_stream
 from app.plex import sync as plex_sync
 from app.plex import watch_sync as plex_watch
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
+from app.routes._common import owned_or_404
 from app.routes.admin import admin_user
 from app.routes.feed import _to_tsquery_str
+from app.utils import iso
 
 log = logging.getLogger("siftlode.plex")