feat(backend): extend UserDevice with MDM health columns

Add mirrored NanoMDM enrollment, supervision and last-seen fields
(mdm_enrolled, mdm_supervised, mdm_last_seen_at) to UserDevice model.
Migration will follow in a separate task.

backend/prisma/schema.prisma

@@ -1113,6 +1113,13 @@ model UserDevice {
     /// NULL → Gerät ist nicht mit einem MDM-UDID verknüpft.
     mdmId String? @map("mdm_id")
 
+    /// Gespiegelter Enrollment-Status aus NanoMDM enrollments.enabled.
+    mdmEnrolled Boolean? @map("mdm_enrolled")
+    /// Gespiegelter Supervision-Status aus NanoMDM devices.unlock_token IS NOT NULL.
+    mdmSupervised Boolean? @map("mdm_supervised")
+    /// Letzter NanoMDM Check-In (enrollments.last_seen_at).
+    mdmLastSeenAt DateTime? @map("mdm_last_seen_at")
+
     @@unique([userId, deviceId])
     @@index([userId])
     @@index([deviceId])