[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/helpdesk_ticket.py

@@ -0,0 +1,11 @@
+from odoo import 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.",
+    )