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

implementation-plans/reboot-command-payload-schemas.md

@@ -0,0 +1,59 @@
+## Reboot Command Payload Schema Snippets
+
+This file provides copy-ready validation snippets for client and integration test helpers.
+
+### Canonical Topics (v1)
+1. Command topic: infoscreen/{client_uuid}/commands
+2. Ack topic: infoscreen/{client_uuid}/commands/ack
+
+### Transitional Compatibility Topics
+1. Command topic alias: infoscreen/{client_uuid}/command
+2. Ack topic alias: infoscreen/{client_uuid}/command/ack
+
+### Canonical Action Values
+1. reboot_host
+2. shutdown_host
+
+### Ack Status Values
+1. accepted
+2. execution_started
+3. completed
+4. failed
+
+### JSON Schema Source
+Use this file for machine validation:
+1. implementation-plans/reboot-command-payload-schemas.json
+
+### Minimal Command Schema Snippet
+```json
+{
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["schema_version", "command_id", "client_uuid", "action", "issued_at", "expires_at", "requested_by", "reason"],
+  "properties": {
+    "schema_version": { "const": "1.0" },
+    "command_id": { "type": "string", "format": "uuid" },
+    "client_uuid": { "type": "string", "format": "uuid" },
+    "action": { "enum": ["reboot_host", "shutdown_host"] },
+    "issued_at": { "type": "string", "format": "date-time" },
+    "expires_at": { "type": "string", "format": "date-time" },
+    "requested_by": { "type": ["integer", "null"] },
+    "reason": { "type": ["string", "null"] }
+  }
+}
+```
+
+### Minimal Ack Schema Snippet
+```json
+{
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["command_id", "status", "error_code", "error_message"],
+  "properties": {
+    "command_id": { "type": "string", "format": "uuid" },
+    "status": { "enum": ["accepted", "execution_started", "completed", "failed"] },
+    "error_code": { "type": ["string", "null"] },
+    "error_message": { "type": ["string", "null"] }
+  }
+}
+```