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/clock_report.py

@@ -166,8 +166,9 @@ class FusionClockReport(models.Model):
         self.attendance_ids = [(6, 0, attendances.ids)]
 
         leave_domain = [
-            ('leave_date', '>=', self.date_start),
+            # Any leave whose range overlaps the report period.
             ('leave_date', '<=', self.date_end),
+            ('date_to', '>=', self.date_start),
         ]
         if self.employee_id:
             leave_domain.append(('employee_id', '=', self.employee_id.id))