fix: default assignment age exclusion (#14998)

## Description
Auto-assignment was skipping the 7-day staleness check entirely for
inboxes that don't have an assignment policy attached. Those inboxes
would pull unassigned conversations of any age off the backlog and hand
them to agents — including conversations untouched for months — while
the activity log still credited "Default Policy" for the assignment.
This makes the default behaviour match what that label implies: with no
policy configured, conversations with no activity in the last 7 days are
now excluded, the same window a freshly created policy uses.

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/models/assignment_policy.rb

@@ -22,6 +22,8 @@
 #  index_assignment_policies_on_enabled              (enabled)
 #
 class AssignmentPolicy < ApplicationRecord
+  DEFAULT_EXCLUDE_OLDER_THAN_HOURS = 168
+
   belongs_to :account
   has_many :inbox_assignment_policies, dependent: :destroy
   has_many :inboxes, through: :inbox_assignment_policies