Add subscriptions to pipeline mini graph stages

What does this MR do and why?

Adds real-time statues to pipeline mini graph stage jobs.

References

Screenshots or screen recordings

How to set up and validate locally

1. Enable flags pipeline_mini_graph_subscription and ci_stage_subscription
2. Visit the pipeline mini graph on Build > Pipelines or pipelines tab on an MR.
3. Ensure web socket delivers status updates (only subscribes when dropdown is open) and unsubscribes after stage is closed.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #515846