fix(backend): address gift card review findings

Fix #1: remove expiry_date from gift card INSERTs (rolling 24-month via last_used_at only)

Fix #2: add FOR UPDATE to RedeemGiftCard SELECT (race condition)

Fix #6: add ?type=customer|inventory filter to GetGiftCards

Fix #7: add admin_audit_log INSERT to GetUserGiftCardBalanceAdmin

Fix #10: refactor normalizeCode to validators.NormalizeGiftCardCode

Schema: add admin_audit_log table (GDPR Article 30), update testdb.go

Tests: expiry-date-null, inventory-filter, audit-log, db table references

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/testutils/testdb/testdb.go

@@ -103,6 +103,7 @@ func Migrate(t *testing.T, pool *pgxpool.Pool) {
 		// Drop all tables, sequences, and views in correct order
 		dropOrder := []string{
 			"till_sales",
+			"admin_audit_log",
 			"gift_card_transactions",
 			"gift_card_expired_balances",
 			"booking_discounts",
@@ -272,6 +273,7 @@ func TruncateTables(t *testing.T, pool *pgxpool.Pool) {
 
 	tables := []string{
 		"till_sales",
+		"admin_audit_log",
 		"gift_card_transactions",
 		"gift_card_expired_balances",
 		"booking_discounts",