fix(aria): add BIGSERIAL PK to device_security_posture, fix JPA persist error

V4 migration drops the device_id primary key and adds an auto-generated
id BIGSERIAL as the PK. device_id becomes a unique nullable column.
Agents that don't send a numeric deviceId can now save posture reports.
Controller GET /{deviceId} now uses findByDeviceId instead of findById.

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

src/main/java/com/hiveops/aria/controller/DevicePostureController.java

@@ -66,7 +66,7 @@ public class DevicePostureController {
     @GetMapping("/{deviceId}")
     @PreAuthorize("hasAnyRole('MSP_ADMIN','BCOS_ADMIN')")
     public ResponseEntity<DeviceSecurityPosture> getPosture(@PathVariable Long deviceId) {
-        return postureRepository.findById(deviceId)
+        return postureRepository.findByDeviceId(deviceId)
                 .map(ResponseEntity::ok)
                 .orElse(ResponseEntity.notFound().build());
     }