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

server/alembic/versions/aa12bb34cc56_add_client_commands_table.py

@@ -0,0 +1,63 @@
+"""add client commands table
+
+Revision ID: aa12bb34cc56
+Revises: f3c4d5e6a7b8
+Create Date: 2026-04-03 12:40:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'aa12bb34cc56'
+down_revision: Union[str, None] = 'f3c4d5e6a7b8'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        'client_commands',
+        sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
+        sa.Column('command_id', sa.String(length=36), nullable=False),
+        sa.Column('client_uuid', sa.String(length=36), nullable=False),
+        sa.Column('action', sa.String(length=32), nullable=False),
+        sa.Column('status', sa.String(length=40), nullable=False),
+        sa.Column('reason', sa.Text(), nullable=True),
+        sa.Column('requested_by', sa.Integer(), nullable=True),
+        sa.Column('issued_at', sa.TIMESTAMP(timezone=True), nullable=False),
+        sa.Column('expires_at', sa.TIMESTAMP(timezone=True), nullable=False),
+        sa.Column('published_at', sa.TIMESTAMP(timezone=True), nullable=True),
+        sa.Column('acked_at', sa.TIMESTAMP(timezone=True), nullable=True),
+        sa.Column('execution_started_at', sa.TIMESTAMP(timezone=True), nullable=True),
+        sa.Column('completed_at', sa.TIMESTAMP(timezone=True), nullable=True),
+        sa.Column('failed_at', sa.TIMESTAMP(timezone=True), nullable=True),
+        sa.Column('error_code', sa.String(length=64), nullable=True),
+        sa.Column('error_message', sa.Text(), nullable=True),
+        sa.Column('created_at', sa.TIMESTAMP(timezone=True), server_default=sa.func.current_timestamp(), nullable=False),
+        sa.Column('updated_at', sa.TIMESTAMP(timezone=True), server_default=sa.func.current_timestamp(), nullable=False),
+        sa.ForeignKeyConstraint(['client_uuid'], ['clients.uuid'], ondelete='CASCADE'),
+        sa.ForeignKeyConstraint(['requested_by'], ['users.id'], ondelete='SET NULL'),
+        sa.PrimaryKeyConstraint('id'),
+        sa.UniqueConstraint('command_id')
+    )
+
+    op.create_index(op.f('ix_client_commands_action'), 'client_commands', ['action'], unique=False)
+    op.create_index(op.f('ix_client_commands_client_uuid'), 'client_commands', ['client_uuid'], unique=False)
+    op.create_index(op.f('ix_client_commands_command_id'), 'client_commands', ['command_id'], unique=False)
+    op.create_index(op.f('ix_client_commands_requested_by'), 'client_commands', ['requested_by'], unique=False)
+    op.create_index(op.f('ix_client_commands_status'), 'client_commands', ['status'], unique=False)
+    op.create_index('ix_client_commands_client_status_created', 'client_commands', ['client_uuid', 'status', 'created_at'], unique=False)
+
+
+def downgrade() -> None:
+    op.drop_index('ix_client_commands_client_status_created', table_name='client_commands')
+    op.drop_index(op.f('ix_client_commands_status'), table_name='client_commands')
+    op.drop_index(op.f('ix_client_commands_requested_by'), table_name='client_commands')
+    op.drop_index(op.f('ix_client_commands_command_id'), table_name='client_commands')
+    op.drop_index(op.f('ix_client_commands_client_uuid'), table_name='client_commands')
+    op.drop_index(op.f('ix_client_commands_action'), table_name='client_commands')
+    op.drop_table('client_commands')