fix(s4): make chain_id nullable for PENDING transfers (STA-134)

ChainTransfer starts in PENDING state with chain_id = null;
it is set during the selectChain() transition to CHAIN_SELECTED.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Puneethkumarck

blockchain-custody/blockchain-custody/src/main/resources/db/migration/V3__fix_chain_id_nullable.sql

@@ -0,0 +1,7 @@
+-- ============================================================
+-- S4 Blockchain & Custody — Fix chain_id nullable for PENDING transfers
+-- ChainTransfer starts in PENDING state with chain_id = null;
+-- chain_id is set during selectChain() transition to CHAIN_SELECTED.
+-- ============================================================
+
+ALTER TABLE chain_transfers ALTER COLUMN chain_id DROP NOT NULL;