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

models/models.py

@@ -147,6 +147,14 @@ class Client(Base):
     screen_health_status = Column(Enum(ScreenHealthStatus), nullable=True, server_default='UNKNOWN')
     last_screenshot_hash = Column(String(32), nullable=True)
 
+    # Systemd service-failed tracking
+    service_failed_at = Column(TIMESTAMP(timezone=True), nullable=True)
+    service_failed_unit = Column(String(128), nullable=True)
+
+    # MQTT broker connection health
+    mqtt_reconnect_count = Column(Integer, nullable=True)
+    mqtt_last_disconnect_at = Column(TIMESTAMP(timezone=True), nullable=True)
+
 
 class ClientLog(Base):
     __tablename__ = 'client_logs'
@@ -164,6 +172,33 @@ class ClientLog(Base):
     )
 
 
+class ClientCommand(Base):
+    __tablename__ = 'client_commands'
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    command_id = Column(String(36), nullable=False, unique=True, index=True)
+    client_uuid = Column(String(36), ForeignKey('clients.uuid', ondelete='CASCADE'), nullable=False, index=True)
+    action = Column(String(32), nullable=False, index=True)
+    status = Column(String(40), nullable=False, index=True)
+    reason = Column(Text, nullable=True)
+    requested_by = Column(Integer, ForeignKey('users.id', ondelete='SET NULL'), nullable=True, index=True)
+    issued_at = Column(TIMESTAMP(timezone=True), nullable=False)
+    expires_at = Column(TIMESTAMP(timezone=True), nullable=False)
+    published_at = Column(TIMESTAMP(timezone=True), nullable=True)
+    acked_at = Column(TIMESTAMP(timezone=True), nullable=True)
+    execution_started_at = Column(TIMESTAMP(timezone=True), nullable=True)
+    completed_at = Column(TIMESTAMP(timezone=True), nullable=True)
+    failed_at = Column(TIMESTAMP(timezone=True), nullable=True)
+    error_code = Column(String(64), nullable=True)
+    error_message = Column(Text, nullable=True)
+    created_at = Column(TIMESTAMP(timezone=True), server_default=func.current_timestamp(), nullable=False)
+    updated_at = Column(TIMESTAMP(timezone=True), server_default=func.current_timestamp(), onupdate=func.current_timestamp(), nullable=False)
+
+    __table_args__ = (
+        Index('ix_client_commands_client_status_created', 'client_uuid', 'status', 'created_at'),
+    )
+
+
 class EventType(enum.Enum):
     presentation = "presentation"
     website = "website"