feat: autotune PSU capacity from the same full-load run as fan ceilings

Extract the fan peak-tracking into observedPeakStore (observe max under
load, hold >= minHold to reject spikes, round, persist JSON) and add a
second instance for PSU draw (psu-observation.json, keyed by PSU
ordinal). Fed from samplePSUPower like fans are from sampleFanSpeeds, so
any full-load run refines it — the Fan Ceiling Check (which also samples
PSU power at a slow cadence off its loop and writes psu_<i>_peak_w), a
burn, thermal cycling, and the 5s web metrics collector.

/topo PSU cards now scale the load fill by wattage_w when the BMC
reports it, else by the observed peak draw — marked "~N% load". This
MSI stand's BMC gives only instantaneous input power, so the observed
peak is the only capacity figure available.

Fan behaviour is unchanged (tests exercise updateFanObservation /
estimateFanDutyCyclePctFromObservation / ResolveFanMaxRPM through the
new store).

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

audit/internal/webui/page_topo_test.go

@@ -262,6 +262,27 @@ func TestFanSpinPeriodSec(t *testing.T) {
 	}
 }
 
+func TestRenderTopoPSURowUsesObservedMaxWhenNoRating(t *testing.T) {
+	ok := "OK"
+	draw := 340.0
+	psus := []schema.HardwarePowerSupply{
+		{HardwareComponentStatus: schema.HardwareComponentStatus{Status: &ok}, InputPowerW: &draw}, // no WattageW
+	}
+	// Observed peak (keyed by ordinal) stands in for the missing nameplate.
+	html := renderTopoPSURow(psus, map[string]float64{"0": 2400})
+	if !strings.Contains(html, `data-psu-max-src="observed"`) {
+		t.Fatalf("expected observed-scale marker: %s", html)
+	}
+	if !strings.Contains(html, "340 / ~2400 W · ~14% load") {
+		t.Fatalf("expected estimated load line: %s", html)
+	}
+	// Without an observed peak either, just the raw watts, no fill.
+	html = renderTopoPSURow(psus, nil)
+	if strings.Contains(html, "% load") {
+		t.Fatalf("no rating and no observed peak → no load figure: %s", html)
+	}
+}
+
 func TestTopoPageRendersStorageDisksGroupedByType(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "audit.json")