Add green/amber/red status indicators and plain-English explanations to dashboard metrics

## Problem

The dashboard currently shows raw numbers — table sizes in MB, query execution time in ms, order counts. An engineer can interpret these, but the person actually responsible for the site (store owner, account manager, non-technical stakeholder) cannot.

There is no at-a-glance answer to "is my site healthy right now?" A non-technical user has to read the numbers, understand what they mean, and make their own judgment. That is the engineer's job, not the client's.

## What to build

Every metric on the dashboard should have:

1. **A status indicator** — green (healthy), amber (watch), red (action needed)
2. **A plain-English explanation** — one sentence that tells the user what the number means and what to do about it (if anything)

**Examples:**

| Metric | Raw value | Status | Plain-English sentence |
|---|---|---|---|
| Order itemmeta size | 4,200 MB | 🟡 Amber | Growing steadily — at this pace, archival will be needed in approximately 3 months. |
| Query execution time | 847ms | 🔴 Red | Slower than your baseline of 310ms — DB load may be affecting checkout speed. |
| Autoloaded options | 2.4 MB | 🟢 Green | Within a healthy range. No action needed. |
| Plugin count | 34 active | 🟡 Amber | Higher than typical. Consider auditing inactive or redundant plugins. |

**Thresholds for each metric** need to be defined as part of this issue (see acceptance criteria).

## Why this matters for selling the product

A store owner or Trevin needs to open the dashboard and immediately know the answer to one question: "Is everything fine?" Right now the answer requires engineering knowledge to derive. With status indicators and plain-English explanations, anyone can read the dashboard and know whether to act.

This is the difference between a tool an engineer uses and a tool a client buys.

## Acceptance criteria

- [ ] Every metric on the dashboard has a green / amber / red status indicator
- [ ] Every metric has a one-sentence plain-English explanation of what the value means
- [ ] Thresholds for each status are defined and documented (either hardcoded sensible defaults or configurable per site)
- [ ] Red status on any metric is immediately visible without scrolling
- [ ] Explanations are written for a non-technical reader — no jargon, no raw units without context

## Related

- #11 — server-side metrics (new metrics from this issue should also have status indicators when added)
- #28 — archival signal banner (done; this issue extends that pattern to all metrics, not just archival)