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).

backend/app/routes/messages.py

@@ -319,15 +319,10 @@ def get_thread(
                 changed = True
         if changed:
             db.commit()
-            # MB3: tell this user's OTHER tabs/devices their unread dropped so the badge updates
-            # live instead of waiting for the next 20s poll. Carries the fresh total so the client
-            # needs no extra fetch.
-            n = db.scalar(
-                select(func.count())
-                .select_from(Message)
-                .where(Message.recipient_id == user.id, Message.read_at.is_(None))
-            )
-            manager.push(user.id, {"type": "unread", "count": int(n or 0)})
+            # MB3: ping this user's OTHER tabs/devices that their unread dropped so the badge
+            # refreshes live instead of waiting for the next 20s poll. The client re-fetches the
+            # count on this signal, so no number is carried here.
+            manager.push(user.id, {"type": "unread"})
     return {"partner": partner, "items": [_serialize_msg(m) for m in msgs]}