fix(plex): make watch import concurrency-safe (upsert), guard toggle

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.

frontend/src/components/SettingsPanel.tsx

@@ -469,7 +469,11 @@ function PlexWatchSync() {
     <Section title={t("settings.plexSync.title")}>
       <p className="text-xs text-muted leading-relaxed mb-2">{t("settings.plexSync.intro")}</p>
       <SettingRow label={t("settings.plexSync.toggle")} hint={t("settings.plexSync.toggleHint")}>
-        <Switch checked={enabled} onChange={(v) => toggle.mutate(v)} />
+        <Switch
+          checked={enabled}
+          disabled={toggle.isPending || reimport.isPending}
+          onChange={(v) => !toggle.isPending && toggle.mutate(v)}
+        />
       </SettingRow>
       {enabled && (
         <div className="mt-2 flex items-center justify-between gap-3">