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/collector/collector.go

@@ -27,6 +27,7 @@ type SystemState struct {
 	PhaseL2PowerW    float64 // per-phase grid power L2
 	PhaseL3PowerW    float64 // per-phase grid power L3
 	WWTopTempC       float64 // DHW cylinder top temperature (°C)
+	PVYieldTodayKWh  float64 // cumulative PV production today (kWh)
 }
 
 // IsExporting returns true if the system is exporting to grid.
@@ -93,6 +94,7 @@ func (c *Collector) Collect(ctx context.Context) (SystemState, error) {
 		{"phase_l2_power", &state.PhaseL2PowerW, 1},
 		{"phase_l3_power", &state.PhaseL3PowerW, 1},
 		{"ww_top_temp", &state.WWTopTempC, 1},
+		{"pv_yield_today", &state.PVYieldTodayKWh, 0.001}, // Wh → kWh
 	}
 
 	for _, t := range targets {
@@ -118,6 +120,7 @@ func (c *Collector) Collect(ctx context.Context) (SystemState, error) {
 		"grid_w", state.GridPowerW,
 		"soc", state.BatterySOC,
 		"pv_w", state.PVProductionW,
+		"pv_today_kwh", fmt.Sprintf("%.1f", state.PVYieldTodayKWh),
 		"compressor_w", state.CompressorPowerW,
 		"ambient_c", state.AmbientTempC,
 		"l1_w", state.PhaseL1PowerW,