feat: pre-render sgdata pages with stale-while-revalidate

[justrach]

Problem

sgdata pages fetch live data on every request. For data that updates every 5-15 minutes (weather, environment readings), there's no need to fetch on every page view.

Proposed Solution

Pre-render sgdata pages on a schedule and serve the pre-rendered HTML instantly. Revalidate in the background when data goes stale.

How it would work

pub const prerender = true;
pub const revalidate = 300; // seconds — re-render every 5 minutes

pub fn render(req: mer.Request) mer.Response {
    const weather = try mer.fetch(req.allocator, .{ .url = "..." });
    return mer.render(req.allocator, page(weather.body));
}

1. On first request: render page, save to dist/, serve HTML
2. On subsequent requests: serve dist/ HTML instantly (0ms render time)
3. After revalidate seconds: next request triggers background re-render
4. Old page served until new render completes (stale-while-revalidate)

Tasks

• Add pub const revalidate: u32 support to route codegen
• Implement background re-render timer in server.zig
• Serve pre-rendered pages from dist/ with freshness check
• For Workers: use Cron Triggers or Durable Objects for scheduled re-render
• Update sgdata pages with revalidate = 300
• Test: first request renders, subsequent requests serve instantly

Expected Impact

LCP: 5.4s → <0.5s for all requests after the first render.
Equivalent to Next.js ISR (Incremental Static Regeneration).