Add historical sparklines to host cards

24h of snapshots were already being retained but never read. Adds a
windowed query (last ~40 samples/host, one query total via
ROW_NUMBER() OVER PARTITION BY, not N+1) embedded in the existing
/api/hosts response, rendered as small hand-rolled SVG sparklines
(cpu/mem/disk overlaid) -- no charting library needed at this scale.

Closes #10.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/src/components/HostCard.tsx

@@ -1,4 +1,5 @@
 import type { HostSummary } from "../api";
+import { Sparkline } from "./Sparkline";
 
 function Bar({ label, pct }: { label: string; pct: number | null }) {
   const value = pct ?? 0;
@@ -34,6 +35,7 @@ export function HostCard({ host }: { host: HostSummary }) {
       <Bar label="CPU" pct={host.cpuPct} />
       <Bar label="Mem" pct={host.memPct} />
       <Bar label="Disk" pct={host.diskPct} />
+      <Sparkline history={host.history} />
       {(host.cpuPressurePct !== null || host.memPressurePct !== null) && (
         <div className="pressure-row">
           {host.cpuPressurePct !== null && <span>CPU pressure {host.cpuPressurePct.toFixed(1)}%</span>}