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>

configs/ems-config.yaml

@@ -108,4 +108,5 @@ ems:
   log_level: "info"
   state_file: "/run/ems/heartbeat"   # written each cycle for state recovery
   recovery_timeout: "1h"             # ignore Shelly state if EMS was down longer
-  override_timeout: "1h"             # how long a manual Shelly change is respected
+  override_timeout: "1h"             # default lockout when EMS detects external Shelly change
+  override_max_import_w: 800         # cancel override immediately if importing more than this (0 = disabled)