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/routers/rooms.py

@@ -201,14 +201,24 @@ async def leave_room_endpoint(
         raise HTTPException(status_code=404, detail="Not a member of this room")
 
 
+def _member_status(user: User, online_ids: set[uuid.UUID]) -> str:
+    # appear_offline always wins, regardless of actual connection -- that's
+    # the whole point of the override (lurking in a room undetected).
+    return "offline" if user.appear_offline or user.id not in online_ids else "online"
+
+
 @router.get("/{room_id}/members", response_model=list[RoomMemberRead])
 async def list_room_members_endpoint(
+    request: Request,
     room_id: uuid.UUID,
     current_user: User = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ):
     await require_room_member(room_id, current_user, db)
     memberships = await list_room_members(db, room_id)
+    online_ids = await request.app.state.global_presence.online_user_ids(
+        [m.user_id for m in memberships]
+    )
     return [
         RoomMemberRead(
             user_id=m.user_id,
@@ -217,6 +227,7 @@ async def list_room_members_endpoint(
             avatar_filename=m.user.avatar_filename,
             role=m.role,
             joined_at=m.joined_at,
+            status=_member_status(m.user, online_ids),
         )
         for m in memberships
     ]
@@ -244,6 +255,7 @@ async def remove_member_endpoint(
 
 @router.patch("/{room_id}/members/{user_id}", response_model=RoomMemberRead)
 async def change_member_role_endpoint(
+    request: Request,
     room_id: uuid.UUID,
     user_id: uuid.UUID,
     data: RoomMemberRoleUpdate,
@@ -259,6 +271,7 @@ async def change_member_role_endpoint(
         raise HTTPException(
             status_code=400, detail="Use transfer-ownership to change the room owner"
         )
+    online_ids = await request.app.state.global_presence.online_user_ids([membership.user_id])
     return RoomMemberRead(
         user_id=membership.user_id,
         username=membership.user.username,
@@ -266,6 +279,7 @@ async def change_member_role_endpoint(
         avatar_filename=membership.user.avatar_filename,
         role=membership.role,
         joined_at=membership.joined_at,
+        status=_member_status(membership.user, online_ids),
     )
 
 
@@ -468,6 +482,7 @@ async def add_member_endpoint(
     except AlreadyMemberError:
         raise HTTPException(status_code=409, detail="That user is already a member")
     await broadcast_room_added(request.app.state.broadcaster, data.user_id, room)
+    online_ids = await request.app.state.global_presence.online_user_ids([membership.user_id])
     return RoomMemberRead(
         user_id=membership.user_id,
         username=membership.user.username,
@@ -475,6 +490,7 @@ async def add_member_endpoint(
         avatar_filename=membership.user.avatar_filename,
         role=membership.role,
         joined_at=membership.joined_at,
+        status=_member_status(membership.user, online_ids),
     )