feat: exclude stale conversations via assignment policy age threshold (#14766)

## Linear ticket
-
https://linear.app/chatwoot/issue/CW-7137/assignment-v2-backlog-flush-overloads-agents

## Description

Assignment policies now skip stale unassigned conversations
automatically. Each policy carries an age threshold (defaults to 7
days), so auto-assignment only picks up recent backlog instead of
draining very old, forgotten conversations. The threshold is
configurable per policy and can be cleared to assign conversations
regardless of age. Previously this control existed only on Enterprise
capacity policies (`exclude_older_than_hours`); it now lives on the
assignment policy itself, so every V2 inbox benefits without needing a
capacity policy.

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

- Local UI flows

## Screenshots? 

<img width="645" height="822" alt="image"
src="https://github.com/user-attachments/assets/ec58db86-c1fa-4f9e-be87-2e24ff02e077"
/>


## 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

---------

Co-authored-by: Sony Mathew <sony@chatwoot.com>

app/javascript/dashboard/i18n/locale/en/settings.json

@@ -798,10 +798,14 @@
         },
         "FAIR_DISTRIBUTION": {
           "LABEL": "Fair distribution policy",
-          "DESCRIPTION": "Set the maximum number of conversations that can be assigned per agent within a time window to avoid overloading any one agent. This required field defaults to 100 conversations per hour.",
+          "DESCRIPTION": "Cap conversations per agent within a time window to avoid overload. Defaults to 100 per hour.",
           "INPUT_MAX": "Assign max",
           "DURATION": "Conversations per agent in every"
         },
+        "EXCLUDE_OLDER_THAN": {
+          "LABEL": "Skip stale conversations",
+          "DESCRIPTION": "Skip unassigned conversations older than this. Defaults to 7 days; clear to disable."
+        },
         "INBOXES": {
           "LABEL": "Added inboxes",
           "DESCRIPTION": "Add inboxes for which this policy will be applicable.",