Add webhook event for issue priority changes

### Problem Statement

When building integrations that sync Sentry issues to external task trackers (Asana, Jira, etc.), manual priority changes in Sentry don't trigger any webhook. The only way to detect priority changes is expensive polling of all open issues.

Issue webhooks fire for: created, resolved, assigned, archived, unresolved. Auto-escalations trigger "unresolved" with substatus "escalating". But manual priority changes produce no webhook event.

### Solution Brainstorm

Add a `priority_changed` action to issue webhooks, or include priority in the payload when it changes alongside other actions.

### Product Area

Settings - Integrations

We want to define a set of [lightweight metrics]() that SDKs can automatically collect for e.g. framework, language or platform, focusing only on signals that **do NOT overlap with Tracing**. Ideally these metrics should give a quick sense of app or runtime health, … while deeper investigation will still rely on traces or profiles.

Examples**:**

* Node.js runtime metrics (e.g., event loop delay, heap usage)
* Prisma-related metrics in Next.js
* Mobile CPU / memory usage to help surface potential ANRs or app hangs

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**Candidate Metrics (add more below)**

* …

*(please also add ideas, questions or examples in comments)*

The issue names no files, tests, or entry points. Start by locating the issue webhook action and payload handling, then inspect existing webhook event tests and how priority changes are recorded. Done means manual priority changes reliably emit a documented webhook event without affecting existing actions.

Written by the indexing model from the issue text.