feat(db): Migration 006 – global_override-Tabelle + override_on_until

server/backend/internal/db/migrations/006_override.sql

@@ -0,0 +1,14 @@
+-- Migration 006: Globaler Override + per-Screen Override-Zeitpunkt
+
+-- Globaler Override: immer maximal eine Zeile (id = 1 per CHECK-Constraint).
+create table if not exists global_override (
+    id      int         primary key default 1,
+    type    text        not null,           -- 'on' oder 'off'
+    until   timestamptz not null,
+    set_at  timestamptz not null default now(),
+    check (id = 1)
+);
+
+-- Per-Screen Override: Einschalten bis Zeitpunkt (null = kein Override).
+alter table screen_schedules
+    add column if not exists override_on_until timestamptz;