feat(fusion_clock): multi-day leave requests (date range)

Request Leave now takes a From/To date range instead of a single day (the To
field is optional -> single-day). Added date_to to fusion.clock.leave.request
(start kept as leave_date), with overlap detection on submit and a date_to >=
leave_date constraint. The absence check and reports now treat a leave as
covering its whole span. The form shows two date inputs; the controller accepts
date_from/date_to (the old single leave_date payload is still honoured). A
migration backfills date_to = leave_date for existing rows.

Live and verified on entech 19.0.3.13.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

fusion_clock/models/hr_attendance.py

@@ -423,7 +423,8 @@ class HrAttendance(models.Model):
 
                     leave = LeaveRequest.search([
                         ('employee_id', '=', emp.id),
-                        ('leave_date', '=', yesterday),
+                        ('leave_date', '<=', yesterday),
+                        ('date_to', '>=', yesterday),
                     ], limit=1)
                     if leave:
                         continue