perf: limit the number of notifications per user to 300 (#13234)

## Linear issue


https://linear.app/chatwoot/issue/CW-6289/limit-the-number-of-notifications-per-user-to-300

## Description

Limits the number of notifications per user to 300 by introducing an
async trim job that runs after each notification creation. This prevents
unbounded notification growth that was causing DB CPU spikes.

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] This change requires a documentation update

## How Has This Been Tested?

- Added unit tests for TrimUserNotificationsJob

## 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

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Implements a dedicated purge job to control notification volume and
scheduling.
> 
> - Introduces `Notification::RemoveOldNotificationJob` (queue:
`purgable`) to delete notifications older than 1 month and trim each
user to the 300 most recent (deterministic by `created_at DESC, id
DESC`)
> - Adds daily cron (`remove_old_notification_job` at 22:30 UTC, queue
`purgable`) in `config/schedule.yml`
> - Removes ad-hoc triggering of the purge from
`TriggerScheduledItemsJob`
> - Adds/updates specs covering enqueue queue, old-notification
deletion, per-user trimming, and combined behavior
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
9ea2b48e36df96cd15d4119d1dd7dcf5250695de. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Vishnu Narayanan <iamwishnu@gmail.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

config/schedule.yml

@@ -59,3 +59,10 @@ periodic_assignment_job:
   cron: '*/30 * * * *'
   class: 'AutoAssignment::PeriodicAssignmentJob'
   queue: scheduled_jobs
+
+# executed daily at 2230 UTC
+# removes old notifications (>1 month) and trims to 300 per user
+remove_old_notification_job:
+  cron: '30 22 * * *'
+  class: 'Notification::RemoveOldNotificationJob'
+  queue: purgable