feat(fusion_clock): add x_fclk_nfc_card_uid to hr.employee

Adds the NFC card UID field (Char, unique, manager-only) that the kiosk
will use to identify employees by card tap. Includes the tests package
with three post-install tests covering write, uniqueness, and nullable
multi-row behaviour.

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

fusion_clock/tests/__init__.py

@@ -0,0 +1,3 @@
+# -*- coding: utf-8 -*-
+
+from . import test_nfc_models