perf(api,frontend): index hot queries, paginate chats, fix realtime

backend/src/api/realtime.py

@@ -65,11 +65,16 @@ class EventHub:
             return
         account_id = event.get("account_id")
         chat_id = event.get("chat_id")
+        scoped = event.get("kind") == "presence"
         targets = [
             sub
             for sub in self._subscribers
             if sub.account_id == account_id
-            and (sub.chat_id is None or sub.chat_id == chat_id)
+            and (
+                sub.chat_id == chat_id
+                if scoped
+                else sub.chat_id is None or sub.chat_id == chat_id
+            )
         ]
         if not targets:
             return