Add source_site_id to ActiveSite model and implement unique constraint. Update serializers and views for site registration and claiming. Enhance tests for site registration and claiming functionality.

backend/apps/sync/migrations/0007_uniq_source_site_id.py

@@ -0,0 +1,21 @@
+# Unique non-empty source_site_id so one farm site cannot own two ActiveSites.
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("sync", "0006_activesite_source_site_id"),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name="activesite",
+            constraint=models.UniqueConstraint(
+                condition=~models.Q(source_site_id=""),
+                fields=("source_site_id",),
+                name="uniq_active_site_source_site_id",
+            ),
+        ),
+    ]