fix(aria): drop NOT NULL on device_id after PK migration

V4 dropped the PK but PostgreSQL retained the implicit NOT NULL.
V5 makes device_id nullable so agents without a numeric deviceId
can persist posture reports.

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

src/main/resources/db/migration/V5__posture_device_id_nullable.sql

@@ -0,0 +1,3 @@
+-- device_id retained its NOT NULL constraint after the PK was dropped in V4.
+-- Make it nullable so agents that don't send a numeric deviceId can persist.
+ALTER TABLE device_security_posture ALTER COLUMN device_id DROP NOT NULL;