[FIX] digest: add missing depends on is_subscribed field

Currently subscribed field is not computed again if user_ids field value
changes on a digest. This has probably few consequences in current code
but better be sure this field is correctly updated in a given transaction.

Task-2641394 (Digest emails sending improvement)
Task-2582128 (Digest onboarding and usage improvement)

Part-of: odoo/odoo#79653

addons/digest/models/digest.py

@@ -41,6 +41,7 @@ class Digest(models.Model):
     kpi_mail_message_total = fields.Boolean('Messages')
     kpi_mail_message_total_value = fields.Integer(compute='_compute_kpi_mail_message_total_value')
 
+    @api.depends('user_ids')
     def _compute_is_subscribed(self):
         for digest in self:
             digest.is_subscribed = self.env.user in digest.user_ids