chore: sync submodule state (parent ref update)

Made-with: Cursor

db/migrations/001_ledger_idempotency.sql

@@ -0,0 +1,7 @@
+-- 001_ledger_idempotency.sql
+-- Add unique constraint for ledger entry idempotency
+-- Prevents duplicate postings with same reference_id per ledger
+
+ALTER TABLE ledger_entries
+  ADD CONSTRAINT ledger_entries_unique_ledger_reference
+  UNIQUE (ledger_id, reference_id);