feat(webui): live-update topology fan tiles without a page reload

Each fan tile carries data-fan / data-ceil / data-measured and a small
script polls /api/metrics/latest every 5s — the metrics collector's own
sampling period, served from memory with no extra BMC call — updating
each tile's spin rate, duty fill and tooltip in place. Polling faster
would only re-read identical numbers; sampling the BMC faster would
choke it under load. FanReading gains json tags for the endpoint.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019VHG21rgTUiR1G3qFHTVmN

audit/internal/platform/sat_fan_stress.go

@@ -32,8 +32,8 @@ type FanCheckOptions struct {
 
 // FanReading holds one fan sensor reading.
 type FanReading struct {
-	Name string
-	RPM  float64
+	Name string  `json:"name"`
+	RPM  float64 `json:"rpm"`
 }
 
 type cachedPowerReading struct {