Add check Queue backlog.

Problem/Motivation

In this Drupal application, background queues (such as core queues or Queue API based workers) may accumulate a large backlog of unprocessed items, which can indicate stuck workers, misconfiguration, or insufficient processing capacity, and can delay critical background tasks.

Steps to reproduce

Add a new Performance FitCheck plugin QueueBacklogCheck that:

• Inspects all defined queues and reads their current item counts (and optionally item age, if available).
• Fails with FitWeight::Medium when any queue exceeds a configurable backlog threshold, listing the affected queues and their sizes so administrators can investigate stuck workers or increase processing.
• Provides a short help message recommending verifying cron/queue workers, tuning queue processing, or scaling worker capacity for heavily used queues.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes