feat: crash recovery, service_failed monitoring, broker health fields, command expiry sweep

- Add GET /api/clients/crashed endpoint (process_status=crashed or stale heartbeat)
- Add restart_app command action with same lifecycle + lockout as reboot_host
- Scheduler: crash auto-recovery loop (CRASH_RECOVERY_ENABLED flag, lockout, MQTT publish)
- Scheduler: unconditional command expiry sweep per poll cycle (sweep_expired_commands)
- Listener: subscribe to infoscreen/+/service_failed; persist service_failed_at + unit
- Listener: extract broker_connection block from health payload; persist reconnect_count + last_disconnect_at
- DB migration b1c2d3e4f5a6: service_failed_at, service_failed_unit, mqtt_reconnect_count, mqtt_last_disconnect_at on clients
- Add GET /api/clients/service_failed and POST /api/clients/<uuid>/clear_service_failed
- Monitoring overview API: include mqtt_reconnect_count + mqtt_last_disconnect_at per client
- Frontend: orange service-failed alert panel (hidden when empty, auto-refresh, quittieren action)
- Frontend: MQTT reconnect count + last disconnect in client detail panel
- MQTT auth hardening: listener/scheduler/server use env credentials; broker enforces allow_anonymous false
- Client command lifecycle foundation: ClientCommand model, reboot_host/shutdown_host, full ACK lifecycle
- Docs: TECH-CHANGELOG, DEV-CHANGELOG, MQTT_EVENT_PAYLOAD_GUIDE, copilot-instructions updated
- Add implementation-plans/, RESTART_VALIDATION_CHECKLIST.md, TODO.md

dashboard/vite.config.ts

@@ -6,6 +6,11 @@ import react from '@vitejs/plugin-react';
 export default defineConfig({
   cacheDir: './.vite',
   plugins: [react()],
+  define: {
+    __BUILD_DATE__: JSON.stringify(new Date().toISOString()),
+    __NODE_VERSION__: JSON.stringify(process.version),
+    __BUILD_ENV__: JSON.stringify(process.env.NODE_ENV ?? 'development'),
+  },
   resolve: {
     // 🔧 KORRIGIERT: Entferne die problematischen Aliases komplett
     // Diese verursachen das "not an absolute path" Problem