feat(ui): queue-health trend card on the maintainer quality dashboard

[JSONbored]

Add a queue-health card to the maintainer quality dashboard rendering pending/in-flight/stuck/DLQ counts + a trend over time from the queue-trend snapshot (#539 maintainer-quality-dashboard). Single card; read-only over the shaped snapshot; shows generatedAt + stale flag per the issue's requirements.

Deliverables

• New QueueHealthCard component in apps/gittensory-ui/src/components/site/app-panels/
• Stats for pending/in-flight/stuck/DLQ + TrendChart (trend-chart.tsx:8) for the queue-depth series
• Render generatedAt timestamp + a 'stale' StatusPill when the snapshot is old (per feat(ui): maintainer repo/org quality dashboard #539 'show generatedAt + a stale flag')
• Wire into the maintainer route (app.maintainer.tsx / MaintainerPanel)
• Vitest test: healthy, stuck>0, DLQ>0, and stale-snapshot branches

References

• apps/gittensory-ui/src/components/site/app-panels/maintainer-panel.tsx:184
• apps/gittensory-ui/src/components/site/trend-chart.tsx:8
• src/services/operator-dashboard.ts:66

Part of #539.

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📸 Required: before/after screenshots (this is a UI change)

Non-negotiable. PRs that change UI must include a before/after screenshot table. PRs without it are automatically closed — no exceptions.

Rules:

• Show every page/feature you changed — before and after, one row each.
• Use the markdown table below. Screenshots go inside the table only — no large full-size images pasted elsewhere in the PR.
• Each cell is a clickable thumbnail (a small image linking to the full-size image) with a one-line caption underneath.
• Upload screenshots to the PR by drag-and-drop (GitHub hosts them on its CDN). Do NOT commit image files to the repo.

Copy this and fill it in (replace FULL_IMAGE_URL with the URL GitHub gives you after you drop the image in):

| Page / Feature | Before | After |
|---|---|---|
| `/your-route` | [<img src="FULL_IMAGE_URL" width="260">](FULL_IMAGE_URL)<br><sub>before: short caption</sub> | [<img src="FULL_IMAGE_URL" width="260">](FULL_IMAGE_URL)<br><sub>after: short caption</sub> |

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2302