docs(readme): refresh screenshots from a seeded demo panel

[nechodom]

What

• Retakes the README Dashboard and Stats screenshots and adds two new ones: the Hostings list and a site detail page (Overview tab). They sit in two 2×2 Markdown tables under "Screenshots".
• bin/hyperion-web/tests/devserver.rs:
  • DEVSERVER_DEMO=1 seeds seven sites (one suspended), 24 h of per-site hosting_usage, 8 h of node_metrics and 1 h of 15 s net_samples, so the charts have something to draw.
  • DEVSERVER_PORT (default 8190) lets it run next to another devpanel.
  • The stub db_create now names databases after the tail of the hosting id. Ids are time-ordered, so sites created in one burst shared a prefix and hit UNIQUE(engine, db_name).
• sqlx added as a dev-dependency of hyperion-web (it is only used by the seed).
• docs/screenshots/README.md documents how to retake the screenshots.

Why

The old screenshots predate the grouped-flat UI and showed an empty node.

Reviewer notes

• Captured from the local devserver, not a real server: headless Chrome, 1600×1000 CSS px at 2×, dark theme.
• The devserver runs on a Mac, so before each capture the page was edited in the DOM:
  • The Mac's hostname and public IP were replaced with node-1 and 203.0.113.10.
  • The "critical services down" and "DNS mismatch" banners were removed and the Services tile was set to OK. Those only reflect a laptop with no nginx and demo domains that don't resolve.
• The seed is test-only code behind an env var; nothing in the shipped binaries changes.
• cargo fmt --all --check and clippy on the devserver target pass.

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