feat: match the Security masthead to the primary-page hero and make History search realtime (#1365)

- Masthead: add `size` and `subtitle` to PageMasthead and render the Security
  masthead at the hero title size with a one-line posture summary, so its height
  and weight match the Home and Fleet mastheads. The compact size with no
  subtitle stays the default, so Settings, Console, and Logs are unchanged.
- History search now applies as you type (a short debounce, no Enter press),
  matching the Fleet overview search. It stays server-side so the query searches
  every completed scan, not just the loaded page.

frontend/src/components/SecurityView.tsx

@@ -171,7 +171,11 @@ export function SecurityView({ activeTab, onTabChange }: SecurityViewProps) {
         state={state}
         tone={tone}
         pulsing={pulsing}
+        size="hero"
         className="rounded-lg mb-4"
+        subtitle={overview
+          ? `${overview.scannedImages} ${overview.scannedImages === 1 ? 'image' : 'images'} scanned · scanner ${overview.scanner.available ? 'ready' : 'not installed'}`
+          : undefined}
         metadata={overview ? [
           { label: 'CRITICAL', value: String(overview.critical), tone: overview.critical > 0 ? 'error' : 'value' },
           { label: 'HIGH', value: String(overview.high), tone: overview.high > 0 ? 'warn' : 'value' },