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/migrations/19.0.3.13.0/post-migrate.py

@@ -0,0 +1,17 @@
+# -*- coding: utf-8 -*-
+# Copyright 2026 Nexa Systems Inc.
+# License OPL-1 (Odoo Proprietary License v1.0)
+"""Backfill leave-request end dates on upgrade to 19.0.3.13.0.
+
+Leave requests gained a `date_to` (end of a multi-day range). Existing
+single-day requests have no end date; set it to the start date so they keep
+being treated as one-day leaves by the absence check and reports.
+"""
+
+
+def migrate(cr, version):
+    if not version:
+        return
+    cr.execute(
+        "UPDATE fusion_clock_leave_request SET date_to = leave_date WHERE date_to IS NULL"
+    )