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>

backend/app/ws/chat.py

@@ -9,6 +9,7 @@ from app.database import get_db
 from app.models import ApiToken, Message, MessageFile, MessageImage, RoomMembership, User
 from app.services.bot_service import resolve_token
 from app.services.message_events import (
+    broadcast_member_updated,
     broadcast_message_update,
     broadcast_new_message,
     broadcast_reaction_update,
@@ -76,9 +77,17 @@ async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)
     await websocket.accept()
     manager = websocket.app.state.connection_manager
     presence = websocket.app.state.presence
+    global_presence = websocket.app.state.global_presence
     broadcaster = websocket.app.state.broadcaster
     joined_rooms: set[uuid.UUID] = set()
     manager.register_user(user.id, websocket)
+    # Only broadcast on a genuine offline->online transition (this user's
+    # first open connection), not for every extra tab -- broadcast_member_
+    # updated tells every room this user's in to refresh, which would be
+    # wasted churn on a transition that didn't actually change anything
+    # visible.
+    if await global_presence.connect(user.id):
+        await broadcast_member_updated(db, broadcaster, user.id)
 
     try:
         while True:
@@ -229,3 +238,5 @@ async def chat_endpoint(websocket: WebSocket, db: AsyncSession = Depends(get_db)
         manager.unregister_user(user.id, websocket)
         for room_id in joined_rooms:
             await presence.leave(room_id, user.id)
+        if await global_presence.disconnect(user.id):
+            await broadcast_member_updated(db, broadcaster, user.id)