Add desktop notification bridge for DS Chat Desktop (#49)

Offline members now also get a desktop_notification WS envelope
alongside the existing Web Push send, since Electron has no push
delivery service configured. The client only acts on it when
window.dsDesktop is present and the user's local preference allows it,
so the server needs no awareness of which clients are Electron.

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

backend/app/services/message_events.py

@@ -55,6 +55,7 @@ async def _notify_offline_members(
         )
 
     room = await db.get(Room, room_id)
+    title = f"#{room.name}" if room else "New message"
     for user_id in offline_ids:
         mentioned = user_id in mentioned_ids
         if message.content:
@@ -64,12 +65,28 @@ async def _notify_offline_members(
             body = f"{sender.username} sent a file"
         else:
             body = f"{sender.username} sent an image"
-        payload = {
-            "title": f"#{room.name}" if room else "New message",
-            "body": body,
-            "room_id": str(room_id),
-        }
+        payload = {"title": title, "body": body, "room_id": str(room_id)}
         await send_push_to_user(db, user_id, payload)
+        # Desktop notifications (#49): delivered over this same already-open
+        # authenticated socket rather than Web Push, since Electron has no
+        # push delivery service configured. Broadcast to every eligible
+        # offline member regardless of push-subscription status -- the
+        # client decides whether to act on it (only when window.dsDesktop
+        # is present), so the server doesn't need to track which clients
+        # are running inside Electron. `id` is the message's own id
+        # (stable, not random) so the client can dedupe across socket
+        # reconnects/replays, the same way Electron's own eventId dedup
+        # does on its side.
+        await broadcaster.publish_to_user(
+            user_id,
+            {
+                "type": "desktop_notification",
+                "id": str(message.id),
+                "room_id": str(room_id),
+                "title": title,
+                "body": body,
+            },
+        )
 
 
 async def _message_payload(db: AsyncSession, message: Message, username: str) -> dict: