feat(mdm): add NanoMDM health columns migration for UserDevice

Adds mdm_id, mdm_enrolled, mdm_supervised, mdm_last_seen_at and an
index on mdm_id. Uses IF NOT EXISTS to stay idempotent because mdm_id
was added manually before the migration was created.

backend/prisma/migrations/20250618_add_mdm_health_columns/migration.sql

@@ -0,0 +1,11 @@
+-- Spiegel-Spalten für NanoMDM Enrollment/Supervision-Status auf UserDevice.
+-- mdm_id wurde manuell hinzugefügt; IF NOT EXISTS macht die Migration idempotent.
+
+ALTER TABLE "rebreak"."user_devices"
+    ADD COLUMN IF NOT EXISTS "mdm_id" TEXT,
+    ADD COLUMN IF NOT EXISTS "mdm_enrolled" BOOLEAN,
+    ADD COLUMN IF NOT EXISTS "mdm_supervised" BOOLEAN,
+    ADD COLUMN IF NOT EXISTS "mdm_last_seen_at" TIMESTAMPTZ(6);
+
+CREATE INDEX IF NOT EXISTS "user_devices_mdm_id_idx"
+    ON "rebreak"."user_devices"("mdm_id");

backend/prisma/schema.prisma

@@ -1123,6 +1123,7 @@ model UserDevice {
     @@unique([userId, deviceId])
     @@index([userId])
     @@index([deviceId])
+    @@index([mdmId])
     @@map("user_devices")
     @@schema("rebreak")
 }