Overhaul system health section with job trigger controls and live timestamps

Rebuild the background jobs card as a sortable table showing each job's
schedule, last run time (live-updating via a new useTimeAgo hook), last
duration, and a manual trigger button backed by a new POST
/api/admin/jobs/trigger route. Extract StatusDot into a shared
component. Add cronToHuman() to display schedule patterns as readable
strings (e.g. "Every 6h", "Daily at 03:00"). Replace static
formatDistanceToNow calls throughout the health section with a
LiveTimeAgo component that refreshes every 30 seconds. Also swap a
handful of section icons for better visual matches across settings cards.

hooks/use-time-ago.ts

@@ -0,0 +1,34 @@
+import { formatDistanceToNowStrict } from "date-fns";
+import { useEffect, useEffectEvent, useState } from "react";
+
+function toTimestamp(date: string | Date | null | undefined): number | null {
+  if (!date) return null;
+  const t = date instanceof Date ? date.getTime() : new Date(date).getTime();
+  return Number.isFinite(t) ? t : null;
+}
+
+export function useTimeAgo(
+  date: string | Date | null | undefined,
+  { intervalMs = 1_000, addSuffix = true, fallback = "" } = {},
+): string {
+  const ts = toTimestamp(date);
+
+  const [text, setText] = useState(() =>
+    ts === null ? fallback : formatDistanceToNowStrict(ts, { addSuffix }),
+  );
+
+  const tick = useEffectEvent(() => {
+    const next =
+      ts === null ? fallback : formatDistanceToNowStrict(ts, { addSuffix });
+    if (next !== text) setText(next);
+  });
+
+  useEffect(() => {
+    tick();
+    if (ts === null) return;
+    const id = setInterval(tick, intervalMs);
+    return () => clearInterval(id);
+  }, [ts, intervalMs]); // tick is NOT listed — that's the point
+
+  return text;
+}