Add presence indicators and a manual "appear offline" override (#36)

Every avatar in the app (chat messages, room member list, your own
avatar in the top bar/profile, the admin user list, the room-invite
search) now shows a green/red presence dot. Also adds a global
"Appear offline" toggle in the account menu, letting a user lurk in a
room undetected -- it overrides the real connection state everywhere,
not per-room.

Backend: new GlobalPresence (backend/app/ws/global_presence.py), a
cross-instance Redis-backed connection tracker parallel to the
existing per-room Presence, incremented/decremented on WS connect/
disconnect. A new users.appear_offline column (migration
f0f6e494454a) always wins over actual connection state when computing
displayed status. RoomMemberRead gained a computed `status` field;
add_member/change_member_role/list_room_members all compute it via a
shared _member_status() helper. Connect/disconnect and profile
updates (display_name, avatar, appear_offline) all broadcast
member_updated to every room the user belongs to, reusing the
broadcast infrastructure from the earlier avatar-staleness fix, so
chat surfaces update live with no new WS envelope type needed. A new
GET /api/users/online gives the admin list and user-search a snapshot
(deliberately not live -- see backend/app/routers/users.py) for
surfaces where "accurate as of page load" is good enough.

Frontend: UserAvatar renders an optional status dot; every call site
threads status/appear_offline through from whichever data source it
already has (room members, the current user, or the new online-ids
snapshot for admin/search).

4 new backend tests (backend/tests/test_presence.py); existing
broadcast-adjacent WS tests updated to tolerate the new member_updated
noise on connect. Verified end-to-end in the browser with two real
users: presence dot flips live on connect/disconnect via the existing
room-broadcast channel, and the lurk toggle correctly forces offline
while still connected.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/AdminPage.tsx

@@ -25,7 +25,7 @@ import {
 } from '../api/admin'
 import { ApiError } from '../api/client'
 import { createApiToken, createBot, listApiTokens, listBots, revokeApiToken } from '../api/bots'
-import { getUserAvatarUrl } from '../api/users'
+import { getUserAvatarUrl, listOnlineUserIds } from '../api/users'
 import { UserPicker } from '../components/UserPicker'
 import { useAuth } from '../context/AuthContext'
 import { hashIndex } from '../lib/avatar'
@@ -55,6 +55,9 @@ export function AdminPage() {
   const { user: currentUser } = useAuth()
   const [tab, setTab] = useState<Tab>('users')
   const [users, setUsers] = useState<AdminUser[]>([])
+  // A snapshot, not live -- see listOnlineUserIds's own comment. Reloaded
+  // whenever the Users tab is opened, same cadence as the user list itself.
+  const [onlineIds, setOnlineIds] = useState<Set<string>>(new Set())
   const [rooms, setRooms] = useState<AdminRoom[]>([])
   const [transferringRoomId, setTransferringRoomId] = useState<string | null>(null)
   const [auditLog, setAuditLog] = useState<AuditLogEntry[]>([])
@@ -98,6 +101,11 @@ export function AdminPage() {
 
   function loadUsers() {
     listAdminUsers().then(setUsers).catch(reportError)
+    listOnlineUserIds()
+      .then((ids) => setOnlineIds(new Set(ids)))
+      .catch(() => {
+        // Non-critical -- the table still works, just without dots.
+      })
   }
 
   function loadRooms() {
@@ -449,6 +457,7 @@ export function AdminPage() {
                       colorIndex={hashIndex(u.username)}
                       size={28}
                       avatarUrl={u.avatar_filename ? getUserAvatarUrl(u.id, u.avatar_filename) : null}
+                      status={onlineIds.has(u.id) ? 'online' : 'offline'}
                     />
                   </td>
                   <td>{u.display_name || u.username}</td>