Add configurable override duration and hard-stop thresholds

Override duration (web UI):
- Dropdown on Ein button: 30min / 1h / 2h / 4h
- Engine notified immediately via ApplyOverride() — no waiting for
  next SyncHardwareState cycle
- Aus button always uses 1h lockout (keeps consumer off for 1h)

Hard-stop thresholds that cancel active overrides:
- SOC emergency brake: now also clears ManualOverride flag so EMS
  resumes full control after the safety shutdown
- override_max_import_w (default 800W): if grid import exceeds this
  while an override is active, override is cancelled immediately
  (no hysteresis delay — protection is instant)

Config: ems.override_max_import_w (0 = disabled)

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

internal/config/config.go

@@ -151,12 +151,13 @@ func (f *ForecastConfig) FetchIntervalParsed() time.Duration {
 
 // EMSConfig holds operational settings for the EMS daemon.
 type EMSConfig struct {
-	PollInterval    string `yaml:"poll_interval"`
-	ListenAddr      string `yaml:"listen_addr"`
-	LogLevel        string `yaml:"log_level"`
-	StateFile       string `yaml:"state_file"`
-	RecoveryTimeout string `yaml:"recovery_timeout"`
-	OverrideTimeout string `yaml:"override_timeout"`
+	PollInterval       string  `yaml:"poll_interval"`
+	ListenAddr         string  `yaml:"listen_addr"`
+	LogLevel           string  `yaml:"log_level"`
+	StateFile          string  `yaml:"state_file"`
+	RecoveryTimeout    string  `yaml:"recovery_timeout"`
+	OverrideTimeout    string  `yaml:"override_timeout"`
+	OverrideMaxImportW float64 `yaml:"override_max_import_w"` // cancel override if grid import exceeds this (0 = disabled)
 }
 
 func (e *EMSConfig) PollIntervalParsed() time.Duration {