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/models/hr_employee.py

@@ -47,6 +47,25 @@ class HrEmployee(models.Model):
         groups="fusion_clock.group_fusion_clock_manager",
     )
 
+    # NFC card (kiosk identification)
+    x_fclk_nfc_card_uid = fields.Char(
+        string='NFC Card UID',
+        index=True,
+        copy=False,
+        groups="fusion_clock.group_fusion_clock_manager",
+        help="Hex UID of the NFC card assigned to this employee. "
+             "Format: uppercase, colon-separated, e.g. 04:A2:B5:62:C1:80. "
+             "Same card the employee uses for door access.",
+    )
+
+    _sql_constraints = [
+        (
+            'fclk_nfc_card_uid_unique',
+            'UNIQUE(x_fclk_nfc_card_uid)',
+            'This NFC card is already assigned to another employee.',
+        ),
+    ]
+
     # On-time streak
     x_fclk_ontime_streak = fields.Integer(
         string='On-Time Streak',