feat(431): TTL-cache health-check results; ?refresh=true forces a fresh run

GET /api/v1/health re-ran all 14 health checks on every request, 4 of
which shell out to ffmpeg/ffprobe via CliWrap — so each poll spawned ~4
subprocesses. The existing HealthCheckSummary cache was write-only.

Cache the full result list for 30s inside HealthCheckService keyed on a
new "healthcheck.results" entry; a non-forced call returns it on a hit,
skipping the checks and the (subscriber-less) summary publish. Add a
`bool forceRefresh` first parameter to IHealthCheckService.PerformHealthChecks:
the API poll path reads the cache, while startup (RunHealthChecksService)
and the troubleshooting support bundle force a fresh run.

Refresh surface: GET /api/v1/health gains an optional `[FromQuery] bool
refresh` (additive, follows the ?deep= exemplar); the SPA "Refresh health"
button calls /api/v1/health?refresh=true, the initial/poll load does not.

Tests: HealthCheckService cache-hit vs force-bypass (mutually opposing,
non-vacuous), handler+controller refresh-flag threading, SPA refresh URL.
Docs: decisions.md 2026-07-19 (#431), api-conventions §2; regenerated v1.json.

fixes #431

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/api/dashboard.ts

@@ -47,8 +47,9 @@ export async function getDashboardData(): Promise<DashboardData> {
   return { channels, channelStates, mediaSources, playouts };
 }
 
-export function getDashboardHealth(): Promise<DashboardHealthCheck[]> {
-  return request<DashboardHealthCheck[]>('/api/v1/health');
+export function getDashboardHealth(refresh = false): Promise<DashboardHealthCheck[]> {
+  // Health results are cached server-side; the on-demand "Refresh health" action forces a fresh run.
+  return request<DashboardHealthCheck[]>(refresh ? '/api/v1/health?refresh=true' : '/api/v1/health');
 }
 
 export function getDashboardVersion(): Promise<DashboardVersion> {
@@ -102,8 +103,8 @@ export function useDashboardHealthQuery(): DashboardHealthQueryState {
     };
   }, []);
 
-  const loadHealth = useCallback(() => {
-    getDashboardHealth()
+  const loadHealth = useCallback((refresh: boolean) => {
+    getDashboardHealth(refresh)
       .then((checks) => {
         if (activeRef.current) {
           setState({ checks, error: null, status: 'success' });
@@ -118,11 +119,11 @@ export function useDashboardHealthQuery(): DashboardHealthQueryState {
 
   const refresh = useCallback(() => {
     setState({ checks: null, error: null, status: 'loading' });
-    loadHealth();
+    loadHealth(true);
   }, [loadHealth]);
 
   useEffect(() => {
-    loadHealth();
+    loadHealth(false);
   }, [loadHealth]);
 
   if (state.status === 'success') {