♻️(backend) align Application model field with is_active convention

Most models in the project use `is_active` rather than `active`.
The Application model was not aligned with this convention.

Rename the field and add a migration to standardize the naming.
Update related tests accordingly.

This change should not introduce breaking changes for external
applications.

src/backend/core/external_api/authentication.py

@@ -203,7 +203,7 @@ class ApplicationJWTAuthentication(BaseJWTAuthentication):
             logger.warning("Application not found: %s", client_id)
             raise exceptions.AuthenticationFailed("Application not found.") from e
 
-        if not application.active:
+        if not application.is_active:
             logger.warning(
                 "Inactive application attempted authentication: %s", client_id
             )