fix: close chat deletion and transport errors

frontend/supabase/migrations/20260720000000_chat_delete_and_dynamic_candidate_confirmation.sql

@@ -0,0 +1,7 @@
+begin;
+drop policy if exists chat_sessions_delete_own on public.chat_sessions;
+create policy chat_sessions_delete_own
+  on public.chat_sessions for delete to authenticated
+  using ((select auth.uid()) = user_id);
+grant delete on table public.chat_sessions to authenticated;
+commit;