Add SG-Ready startup grace period and daily PV yield tracking

SG-Ready startup grace period (sg_ready_startup_min: 10):
Heat pump compressor takes ~9 minutes to start after receiving the
SG-Ready signal. Previously, idle detection fired after only 3×2min=6min,
turning off SG-Ready before the compressor had time to respond. The grace
period suppresses idle detection for the configured duration after activation.
0 = disabled (tests and unconfigured deployments are unaffected).

Daily PV yield (photovoltaic_production_cumulated_currentDay):
Reads actual kWh produced today from Prometheus (Wh → kWh, ÷1000).
Logged as pv_today_kwh each cycle. Shown on status page in two places:
- PV card sub-line: "Heute: 31.9 kWh"
- Forecast card alongside forecast: "Ist: 31.9 kWh"
Immediately makes forecast vs reality visible (today: forecast 15.9,
actual 31.9 kWh — explains why SOC bypass triggered).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/engine/engine.go

@@ -169,8 +169,20 @@ func (e *Engine) Decide(state collector.SystemState, now time.Time, wwBoostC, fo
 	}
 
 	// --- Compressor idle: release SG-Ready early if heat pump stopped ---
+	// A startup grace period is applied after activation: the heat pump compressor
+	// takes several minutes to start after receiving the SG-Ready signal, so idle
+	// detection is suppressed until the grace period has elapsed.
 	if cs := e.consumers[ConsumerSGReady]; cs.Active {
-		if state.CompressorPowerW < float64(e.cfg.Consumers.CompressorIdleW) {
+		startupMin := e.cfg.Consumers.SGReadyStartupMin
+		inGrace := startupMin > 0 && !cs.ActivatedAt.IsZero() && now.Sub(cs.ActivatedAt) < time.Duration(startupMin)*time.Minute
+
+		if inGrace {
+			e.logger.Debug("SG-Ready: startup grace period, skipping idle check",
+				"activated_at", cs.ActivatedAt.Format("15:04"),
+				"grace_min", startupMin,
+				"elapsed_min", int(now.Sub(cs.ActivatedAt).Minutes()),
+			)
+		} else if state.CompressorPowerW < float64(e.cfg.Consumers.CompressorIdleW) {
 			cs.LowPowerCycles++
 			e.logger.Debug("SG-Ready: compressor idle cycle",
 				"compressor_w", state.CompressorPowerW,