fix(deferred): address /code-review findings on the closeout diff

- channels CB3: sort discovery rows NULLs-last (title is None) to match the DB
  ORDER BY exactly — coercing NULL title to "" floated untitled channels to the top.
- AdminUsers SC1: track in-flight rows in a Set, not a single id — per-row disabling
  now lets the admin start concurrent row actions, and a shared id was cleared by
  whichever settled first (re-enabling a still-pending row → possible double-submit).
- ChatThread CT4: reset the incoming-count ref when partnerId changes — the Messages
  page reuses one ChatThread across conversation switches, so a same-count switch
  could skip the open-marks-read badge refresh.
- messages MB3: push a bare {type:"unread"} (drop the now-dead count query — the
  client re-fetches on the signal and ignored the number anyway).
- api.ts: extract the shared keepalive `beacon()` helper (saveProgressBeacon +
  plexProgressBeacon were near-verbatim copies).

frontend/src/components/ChatThread.tsx

@@ -68,8 +68,16 @@ export default function ChatThread({
   // refresh the badges — but ONLY when the incoming-message count actually changed, not on every
   // 20s poll (which would needlessly refetch /conversations + /unread_count each tick).
   const lastIncoming = useRef(-1);
+  const lastPartner = useRef(partnerId);
   useEffect(() => {
     if (!q.dataUpdatedAt) return;
+    // The Messages page reuses one ChatThread instance across conversation switches (not keyed by
+    // partnerId), so reset the counter when the partner changes — otherwise switching to a thread
+    // with the same incoming-count would skip the open-marks-read refresh.
+    if (lastPartner.current !== partnerId) {
+      lastPartner.current = partnerId;
+      lastIncoming.current = -1;
+    }
     const incoming = (q.data?.items ?? []).filter((m) => m.recipient_id === meId).length;
     if (incoming !== lastIncoming.current) {
       lastIncoming.current = incoming;
@@ -77,7 +85,7 @@ export default function ChatThread({
       qc.invalidateQueries({ queryKey: ["message-unread"] });
     }
     // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [q.dataUpdatedAt, meId, qc]);
+  }, [q.dataUpdatedAt, partnerId, meId, qc]);
 
   // Scroll to the newest message when the count changes (open / new message) — NOT on `plain`,
   // which gets a fresh object on every poll's decrypt pass and would yank a scrolled-up reader