fix(conversations): prefetch support_identity_snapshot

conversation_payload для support-диалогов дёргает support_identity_snapshot FK
на каждую строку списка inbox → N+1. Добавлен select_related в
conversations_for_organization. Для sales (snapshot=null) безвредно.

apps/backend/hub_platform/conversations/selectors.py

@@ -6,7 +6,14 @@ from hub_platform.conversations.models import Conversation
 def conversations_for_organization(organization_id: int) -> QuerySet[Conversation]:
     return (
         Conversation.objects.filter(organization_id=organization_id)
-        .select_related("channel", "channel__product", "contact", "connection", "assigned_operator")
+        .select_related(
+            "channel",
+            "channel__product",
+            "contact",
+            "connection",
+            "assigned_operator",
+            "support_identity_snapshot",
+        )
         .order_by("-last_activity_at")
     )