feat: switch image fetcher + og:image scheduler job + dashboard thumbnail column
- Add switch-image-fetcher.ts: og:image-based image discovery for all 86 seeded switches
(covers Cisco, Arista, Juniper, NVIDIA, Edgecore, Celestica, Asterfusion, Dell,
HPE/Aruba, Huawei, Nokia, Extreme, MikroTik, Ubiquiti, FS.COM, Supermicro)
- Wire fetchSwitchImages() into scheduler as scrape:images:switches (daily 08:30 UTC)
- Dashboard: add 48px thumbnail column to switch table (lazy img with gear icon fallback)
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