♻️(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/models.py

@@ -759,7 +759,7 @@ class Application(BaseModel):
         verbose_name=_("Application name"),
         help_text=_("Descriptive name for this application."),
     )
-    active = models.BooleanField(default=True)
+    is_active = models.BooleanField(default=True)
     client_id = models.CharField(
         max_length=100, unique=True, default=utils.generate_client_id
     )