fix: staging CI deploy not triggering on PR reopen

## 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:

```yaml
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`

Start with the staging.yml workflow and inspect its pull_request triggers for opened, synchronize, and reopened events. Check the LIVEPEER_CI_DEPLOY_KEY repository secret against the current deploy key for the recreated livepeer-ci Studio subgraph. Done means reopened or updated PRs trigger staging deployment and CI no longer reports that a deploy key is missing.

Written by the indexing model from the issue text.