The Plex watch import inserted plex_states via per-row ORM add against a pre-read 'existing' set,
which is not safe when the enable call fires more than once while the multi-second import runs (the
toggle wasn't disabled during it): overlapping imports raced to INSERT the same (user_id, item_id)
rows and all but the first hit a UniqueViolation -> 500 (seen on prod). Rewrite the write path as a
chunked PostgreSQL UPSERT (on_conflict_do_update, rows de-duped by item_id) — idempotent and
concurrency-safe, Plex still wins on the intersection and Siftlode-only states are untouched.
Also disable the Settings toggle (and ignore its onChange) while a sync mutation is pending, so it
can't be fired repeatedly; adds a reusable 'disabled' prop to the Switch primitive.
Plex records watch state per Plex account; Siftlode per user in plex_states. A new plex_link table
maps a Siftlode user to a Plex account; for the owner (MVP) the row uses the server admin token
(uses_admin=True), so no separate Plex login. app/plex/watch_sync.py reads the owner account's
viewCount/viewOffset/lastViewedAt (already present in the catalog mirror's section listing) and
upserts them into the owner's plex_states — 'Plex is master' on this first import, but only where
Plex has a watch record (Siftlode-only states are preserved; union on the intersection). Idempotent.
Admin routes: GET/POST /api/plex/watch/link (status + enable/disable; first enable runs the import)
and POST /api/plex/watch/import (re-run). Migration 0051_plex_link. Two-way push (Phase B) and the
incremental Plex→Siftlode reconcile (Phase C) build on this in later ships.
