Fix DM presence indicator never updating live (#63)

The sidebar shows every DM's online/offline dot at once, but the only
existing signal for a presence change (member_updated) is broadcast to
a room's own channel, which Presence only delivers to a connection that
currently has that specific room joined -- never true for a DM sitting
unopened in the sidebar. Add a dedicated per-user broadcast
(dm_presence_update) sent to each of a user's DM partners on their own
per-user channel whenever their global online/offline state changes, so
the sidebar dot updates without needing that DM to be the open room.

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

backend/app/services/message_events.py

@@ -8,6 +8,7 @@ from app.models import Message, MessageFile, MessageMention, Room, RoomMembershi
 from app.schemas.message import ReactionSummary
 from app.services.link_preview_service import fetch_and_broadcast_link_preview
 from app.services.push_service import send_push_to_user
+from app.services.room_service import list_dm_partner_ids
 from app.services.webhook_service import dispatch_event
 from app.ws.broadcaster import Broadcaster
 from app.ws.focus_presence import FocusPresence
@@ -237,6 +238,29 @@ async def broadcast_member_updated(db: AsyncSession, broadcaster: Broadcaster, u
         )
 
 
+async def broadcast_dm_presence_update(
+    db: AsyncSession, broadcaster: Broadcaster, user_id: uuid.UUID, online: bool
+) -> None:
+    """Tells every one of user_id's DM partners that their online/offline
+    status just changed (#63) -- on each partner's own per-user channel,
+    not the DM room's channel. The room channel alone doesn't reach the
+    sidebar: Presence gates room-channel delivery on actually having that
+    specific room's channel joined right now, which is only ever the one
+    room currently open in the UI -- so a DM sitting unopened in the
+    sidebar (which is the normal case; the sidebar shows every DM's status
+    at once) never saw its partner's status change until something else
+    forced a full room-list refetch."""
+    for partner_id in await list_dm_partner_ids(db, user_id):
+        await broadcaster.publish_to_user(
+            partner_id,
+            {
+                "type": "dm_presence_update",
+                "user_id": str(user_id),
+                "status": "online" if online else "offline",
+            },
+        )
+
+
 async def broadcast_room_added(broadcaster: Broadcaster, user_id: uuid.UUID, room: Room) -> None:
     """The only signal a user's open client gets that they were just added
     to a room -- without it, GET /rooms/mine is only ever fetched once at

backend/app/services/room_service.py

@@ -244,6 +244,27 @@ async def list_member_rooms(
     ]
 
 
+async def list_dm_partner_ids(db: AsyncSession, user_id: uuid.UUID) -> list[uuid.UUID]:
+    """Every user this user_id shares a DM with (#63) -- used to know who
+    needs telling about a global online/offline transition, since Presence
+    gates room-channel delivery on actually having that specific room
+    joined right now (only ever the one room currently open in the UI), so
+    a DM sitting unopened in the sidebar would otherwise never hear about
+    its partner's status changing at all."""
+    result = await db.execute(
+        select(RoomMembership.user_id)
+        .join(Room, Room.id == RoomMembership.room_id)
+        .where(
+            Room.is_dm.is_(True),
+            RoomMembership.user_id != user_id,
+            RoomMembership.room_id.in_(
+                select(RoomMembership.room_id).where(RoomMembership.user_id == user_id)
+            ),
+        )
+    )
+    return [row[0] for row in result.all()]
+
+
 async def get_room(db: AsyncSession, room_id: uuid.UUID) -> Room:
     room = await db.get(Room, room_id)
     if room is None: