[ADD] helpdesk_ticket_deadline: add a manual deadline on tickets

This module adds a simple, manually filled ``date_deadline``
Date field, tracked in the chatter, with an overdue flag,
a red kanban decoration and search filters (overdue, due
today, group by deadline).

helpdesk_ticket_deadline/models/__init__.py

@@ -0,0 +1 @@
+from . import helpdesk_ticket

helpdesk_ticket_deadline/models/helpdesk_ticket.py

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+from odoo import api, fields, models
+
+
+class HelpdeskTicket(models.Model):
+    _inherit = "helpdesk.ticket"
+
+    date_deadline = fields.Date(
+        string="Deadline",
+        tracking=True,
+        help="Target date by which the ticket should be resolved.",
+    )
+    deadline_overdue = fields.Boolean(
+        string="Overdue",
+        compute="_compute_deadline_overdue",
+        help="True when the deadline is strictly before today.",
+    )
+
+    @api.depends("date_deadline")
+    def _compute_deadline_overdue(self):
+        """Flag tickets whose deadline is strictly before today.
+
+        A ticket without a deadline is never overdue. The comparison uses
+        the current user's timezone so the flag flips at local midnight.
+        """
+        today = fields.Date.context_today(self)
+        for ticket in self:
+            ticket.deadline_overdue = bool(
+                ticket.date_deadline and ticket.date_deadline < today
+            )