feat(db): screen_schedules-Tabelle für Zeitplan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/backend/internal/db/migrations/005_screen_schedules.sql

@@ -0,0 +1,8 @@
+-- Migration 005: Display-Steuerung – Zeitplan pro Screen
+
+create table if not exists screen_schedules (
+  screen_id       text    primary key references screens(id) on delete cascade,
+  schedule_enabled boolean not null default false,
+  power_on_time   text    not null default '',
+  power_off_time  text    not null default ''
+);