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/api/src/db/index.ts

@@ -101,8 +101,35 @@ export function getLatestSnapshots(db: Database.Database): LatestRow[] {
     .all() as LatestRow[];
 }
 
-// Old snapshots aren't useful yet (no charting in Phase 1) and would grow unbounded
-// on a poll-every-30s cadence, so trim anything older than the retention window.
+export interface HistoryPoint {
+  host_id: string;
+  ts: string;
+  cpu_pct: number | null;
+  mem_pct: number | null;
+  disk_pct: number | null;
+}
+
+// Last N samples per host (not a time window) — at the default 30s poll interval
+// that's ~20 minutes of trend, enough for a sparkline without needing to
+// downsample. A window function keeps this to one query instead of one per host.
+export function getRecentHistory(db: Database.Database, samplesPerHost = 40): HistoryPoint[] {
+  return db
+    .prepare(
+      `
+      SELECT host_id, ts, cpu_pct, mem_pct, disk_pct FROM (
+        SELECT host_id, ts, cpu_pct, mem_pct, disk_pct,
+               ROW_NUMBER() OVER (PARTITION BY host_id ORDER BY id DESC) AS rn
+        FROM metric_snapshots
+      )
+      WHERE rn <= @samplesPerHost
+      ORDER BY host_id, ts ASC
+    `
+    )
+    .all({ samplesPerHost }) as HistoryPoint[];
+}
+
+// Sparklines only ever read the last ~40 samples (see getRecentHistory), so
+// anything older than the retention window is just dead weight — trim it.
 export function pruneOldSnapshots(db: Database.Database, retentionHours: number): void {
   db.prepare(`DELETE FROM metric_snapshots WHERE ts < datetime('now', @cutoff)`).run({
     cutoff: `-${retentionHours} hours`,