Send external notification when archival signal fires

## Context

The archival signal is now evaluated and surfaced as a dashboard banner (#28, #33). However, the banner is only visible when someone actively logs into the WordPress admin and opens the ProPerf dashboard. In normal operations between engagements, nobody is watching — so the signal goes unnoticed until it's too late.

## Problem

The plugin needs to push a notification outside WordPress when the archival signal fires, so the right person is alerted proactively without having to log in and check.

## What to build

When `archival_signal_active` evaluates to `true` during a metrics collection run, send a notification to the configured platform. The notification should include:
- The site URL
- Current order itemmeta size (MB)
- Configured threshold (MB)
- Number of orders older than the retention window

The notification platform and recipient(s) are TBD — pending confirmation from MFM's team (see acceptance criteria).

## Dependency

Depends on #33 — `archival_signal_active` must be evaluated before this notification can be triggered.

## Acceptance criteria

- [x] Confirm with Kuldeep and Deepak which notification platform MFM already uses (Slack, email, SMS, or other)
- [x] Confirm who should receive the alert — specific person or team channel
- [x] Implement notification delivery to the confirmed platform when `archival_signal_active` is `true`
- [ ] Notification is not sent if no threshold is configured
- [x] Notification is not sent on every push if the signal is already active — only fires once when the signal transitions from inactive to active (no alert spam)
- [ ] Notification includes site URL, current itemmeta size, threshold, and archivable order count

## Related

- #28 — dashboard archival signal banner (done)
- #33 — signal evaluation and BigQuery push (done)