Add ciStageStatusUpdated GraphQL subscription for real-time stage updates

What does this MR do and why?

This MR adds a new GraphQL subscription ciStageStatusUpdated that triggers when a CI stage's status changes. This is the backend foundation for real-time pipeline stage status updates on the MR Pipelines page.

Problem

Currently, the MR pipeline mini graph only updates stage statuses through polling or page refresh. While we have real-time updates at the pipeline level (via ciPipelineStatusUpdated), individual stage status transitions (e.g., pending → running → success) are not pushed to the client in real-time.

Solution

Introduce a per-stage GraphQL subscription that triggers on every stage status transition. This allows the frontend to subscribe only to "alive" stages and receive immediate updates when their status changes, providing a more responsive user experience without increasing polling frequency.

Why per-stage instead of per-pipeline?

• Reduced payload size: Sending only the changed stage (~200 bytes) vs all stages (~400+ bytes)
• Fewer DB queries: Only fetch the single stage that changed
• Granular subscriptions: Frontend can subscribe/unsubscribe to individual stages based on their status

References

• Issue: #591338
• Parent epic: &14144
• Frontend Draft MR: !224757 (closed)

How to set up and validate locally

1. Navigate to any merge request and run pipeline
2. Get the stage ID of the last stage of that pipeline (you can find this in Rails console):

   # Find a stage from a running pipeline
   pipeline = Ci::Pipeline.running.last
   stage = pipeline.stages.last
   stage.to_gid.to_s
   # => "gid://gitlab/Ci::Stage/12345"

3. Visit http://gdk.test:3000/-/graphql-explorer and start the subscription:

   subscription {
     ciStageStatusUpdated(stageId: "gid://gitlab/Ci::Stage/12345") {
       id
       name
       detailedStatus {
         icon
         name
         tooltip
       }
     }
   }

4. Wait for the stage to complete all the jobs
5. Watch the subscription data update in the GraphQL explorer

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