feat(db): screen_status-Tabelle für Display-Zustand

server/backend/internal/db/migrations/004_screen_status.sql

@@ -0,0 +1,8 @@
+-- Migration 004: Display-Steuerung – screen_status
+-- Speichert den zuletzt vom Agent gemeldeten Display-Zustand pro Screen.
+
+create table if not exists screen_status (
+  screen_id     text         primary key references screens(id) on delete cascade,
+  display_state text         not null default 'unknown',
+  reported_at   timestamptz  not null default now()
+);