Multi-site onboarding runbook and unified client view

## Problem

The plugin currently works on one site (MFM). To sell this as a standalone product, a new client needs to be fully onboarded — plugin installed, BigQuery configured, data flowing — in under 30 minutes. There is no documented process for this, and there is no way to see all client sites in one place.

Without this, the product cannot scale beyond the first client.

## What to build

### 1. Onboarding runbook

A step-by-step document covering the full onboarding flow for a new client:

1. Download and install the plugin ZIP
2. Create a BigQuery service account and generate credentials
3. Configure credentials under ProPerf → Settings
4. Verify first push succeeds
5. Set the DB size alert threshold (with auto-suggestion)
6. Confirm dashboard is showing live data

The runbook should be completable by someone who knows WordPress but is not a backend engineer.

### 2. Unified multi-site view

A way to see all onboarded client sites in one place — even a minimal implementation:

- A BigQuery view or Looker Studio page that aggregates `site_url`, latest `archival_signal_active`, latest `order_itemmeta_size_mb`, and `timestamp_utc` across all sites
- Makes it possible to monitor 5+ clients without opening each WordPress admin individually

## Why this matters for selling the product

A second client cannot be onboarded without a repeatable process. The onboarding runbook is what makes the sale completable — without it, every new client requires the same ad-hoc setup work. The unified view is what makes managing multiple clients practical.

## Acceptance criteria

- [ ] Onboarding runbook documented and covers all steps from ZIP install to first successful BigQuery push
- [ ] Runbook is completable without SSH or WP-CLI access
- [ ] BigQuery view or Looker Studio page exists showing all sites with latest signal status and itemmeta size
- [ ] A new client can be fully onboarded in under 30 minutes following the runbook

## Related

- #19 — build script (done; onboarding runbook builds on this)
- #11 — server-side metrics (once collected, these should appear in the unified view too)