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/AdminUsers.tsx

@@ -52,25 +52,32 @@ function UsersRoles({ me }: { me: Me }) {
   const qc = useQueryClient();
   const confirm = useConfirm();
   const q = useQuery({ queryKey: ["admin-users"], queryFn: api.adminUsers });
-  // Which row currently has an action in flight, so we disable ONLY that row's button (mutations
-  // are single-flight, so one id suffices) rather than greying every row (SC1).
-  const [pendingId, setPendingId] = useState<number | null>(null);
+  // Which rows have an action in flight, so we disable ONLY those rows' buttons (SC1) — a Set, since
+  // with per-row disabling the admin can now start actions on several rows concurrently.
+  const [pendingIds, setPendingIds] = useState<Set<number>>(new Set());
+  const startPending = (id: number) => setPendingIds((s) => new Set(s).add(id));
+  const endPending = (id: number) =>
+    setPendingIds((s) => {
+      const n = new Set(s);
+      n.delete(id);
+      return n;
+    });
   const setRole = useMutation({
     mutationFn: ({ id, role }: { id: number; role: "user" | "admin"; email: string }) =>
       api.setUserRole(id, role),
-    onMutate: (vars) => setPendingId(vars.id),
+    onMutate: (vars) => startPending(vars.id),
     onSuccess: (_d, vars) => {
       qc.invalidateQueries({ queryKey: ["admin-users"] });
       notify({ level: "success", message: t("users.roles.updated", { email: vars.email }) });
     },
-    onSettled: () => setPendingId(null),
+    onSettled: (_d, _e, vars) => endPending(vars.id),
     // Errors (e.g. last-admin / self) surface via the global error dialog with the server's detail.
   });
 
   const suspend = useMutation({
     mutationFn: ({ id, suspended }: { id: number; suspended: boolean; email: string }) =>
       api.setUserSuspended(id, suspended),
-    onMutate: (vars) => setPendingId(vars.id),
+    onMutate: (vars) => startPending(vars.id),
     onSuccess: (_d, vars) => {
       qc.invalidateQueries({ queryKey: ["admin-users"] });
       notify({
@@ -80,17 +87,17 @@ function UsersRoles({ me }: { me: Me }) {
         }),
       });
     },
-    onSettled: () => setPendingId(null),
+    onSettled: (_d, _e, vars) => endPending(vars.id),
     // Guards (demo / self / last admin) surface via the global error dialog.
   });
   const del = useMutation({
     mutationFn: ({ id }: { id: number; email: string }) => api.adminDeleteUser(id),
-    onMutate: (vars) => setPendingId(vars.id),
+    onMutate: (vars) => startPending(vars.id),
     onSuccess: (_d, vars) => {
       qc.invalidateQueries({ queryKey: ["admin-users"] });
       notify({ level: "success", message: t("users.delete.done", { email: vars.email }) });
     },
-    onSettled: () => setPendingId(null),
+    onSettled: (_d, _e, vars) => endPending(vars.id),
   });
 
   const onToggle = async (u: AdminUserRow) => {
@@ -171,7 +178,7 @@ function UsersRoles({ me }: { me: Me }) {
                 >
                   <button
                     onClick={() => onToggle(u)}
-                    disabled={u.is_demo || self || pendingId === u.id}
+                    disabled={u.is_demo || self || pendingIds.has(u.id)}
                     className="shrink-0 glass-card glass-hover inline-flex items-center gap-1 px-2.5 py-1.5 rounded-lg text-xs disabled:opacity-40 transition"
                   >
                     <Shield className="w-3.5 h-3.5" />
@@ -191,7 +198,7 @@ function UsersRoles({ me }: { me: Me }) {
                 >
                   <button
                     onClick={() => onSuspend(u)}
-                    disabled={u.is_demo || self || pendingId === u.id}
+                    disabled={u.is_demo || self || pendingIds.has(u.id)}
                     aria-label={u.is_suspended ? t("users.suspend.undoAction") : t("users.suspend.action")}
                     className={`shrink-0 p-1.5 rounded-lg border disabled:opacity-40 transition ${
                       u.is_suspended
@@ -205,7 +212,7 @@ function UsersRoles({ me }: { me: Me }) {
                 <Tooltip hint={u.is_demo || self ? t("users.delete.locked") : t("users.delete.action")}>
                   <button
                     onClick={() => onDelete(u)}
-                    disabled={u.is_demo || self || pendingId === u.id}
+                    disabled={u.is_demo || self || pendingIds.has(u.id)}
                     aria-label={t("users.delete.action")}
                     className="shrink-0 p-1.5 rounded-lg border border-border text-muted hover:text-red-400 disabled:opacity-40 transition"
                   >