fix: staging CI deploy not triggering on PR reopen

[rickstaa]

Problem

The staging CI workflow (staging.yml) does not seem to deploy when a PR is reopened or updated. The deploy key secret (LIVEPEER_CI_DEPLOY_KEY) may also need to be refreshed if the livepeer-ci Studio subgraph was recreated.

Suggested Fix

1. Explicitly add PR event types to ensure the workflow triggers on reopen/update:

on:
  pull_request:
    types: [opened, synchronize, reopened]
    branches:
      - "*"

2. Verify the LIVEPEER_CI_DEPLOY_KEY repo secret matches the current deploy key in Subgraph Studio.

Context

• The livepeer-ci subgraph in Studio appears to have been recreated, which would invalidate the old deploy key.
• CI currently errors with: You must provide a deploy key