♻️(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/migrations/0018_rename_active_application_is_active.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.12 on 2026-03-11 14:39
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0017_file'),
+    ]
+
+    operations = [
+        migrations.RenameField(
+            model_name='application',
+            old_name='active',
+            new_name='is_active',
+        ),
+    ]