perf: sync protocol and core hardening updates

deploy/migrations/0074_dispatch_outbox_poison_head_index.up.sql

@@ -0,0 +1,10 @@
+-- Terminal failed rows are short-lived diagnostic quarantine entries.  Cleanup starts from the
+-- one-row-per-user durable head so it can lock in the same head→outbox order as claim/completion
+-- without scanning healthy lanes.
+CREATE INDEX dispatch_outbox_user_heads_failed_cleanup_idx
+    ON public.dispatch_outbox_user_heads (
+        updated_at,
+        target_user_id,
+        head_id
+    )
+    WHERE status = 'failed';