fix: serialise redirect writes so concurrent saves cannot store a loop (WR-10)

The save-time simulation read the rules at READ COMMITTED without a lock,
so A -> B and B -> A written concurrently both passed. CheckRedirect,
AddressChanged and every batch now take a transaction-level advisory lock
before they read the rules.

classes/addresses.go

@@ -96,6 +96,9 @@ func (s *Service) applyAddressRedirects(ctx context.Context, tx *gorm.DB, res *A
 		return nil
 	}
 
+	if err := lockRedirectWrites(tx.WithContext(ctx)); err != nil {
+		return err
+	}
 	rules, err := LoadRedirects(tx)
 	if err != nil {
 		return err