feat: add bounded KIS account read recovery

trex_core/adapters/persistence/sql/021_create_broker_account_snapshot.sql

@@ -0,0 +1,10 @@
+-- dcy-041 / GitHub #45: account-scoped last healthy KIS snapshot.
+-- account_hash is SHA-256(CANO|ACNT_PRDT_CD); no plaintext account identity,
+-- token, key, order response, or broker payload is persisted.
+CREATE TABLE IF NOT EXISTS broker_account_snapshot (
+    account_hash    TEXT PRIMARY KEY,
+    holdings        JSONB NOT NULL,
+    holdings_captured_at TIMESTAMPTZ,
+    available_cash  BIGINT CHECK (available_cash IS NULL OR available_cash >= 0),
+    cash_captured_at TIMESTAMPTZ
+);