[IMP] website_slides: add constraint completion to be a percentage

Before completion could hold about any value, even if it is generally correctly
computed as a percentage. To be sure we don't have incoherent values in db
a constraint is added to ensure completion is contained between 0 and 100.

--- Links ---

Task-2777216
Prepares Task-2508019 (Slides membership refactoring)

Also see UPG PR - odoo/upgrade#2854

Part-of: odoo/odoo#74171

addons/website_slides/models/slide_channel.py

@@ -39,6 +39,10 @@ class ChannelUsersRelation(models.Model):
          'unique(channel_id, partner_id)',
          'A partner membership to a channel must be unique!'
         ),
+        ('check_completion',
+         'check(completion >= 0 and completion <= 100)',
+         'The completion of a channel is a percentage and should be between 0% and 100.'
+        )
     ]
 
     def _recompute_completion(self):