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/0006_activesite_source_site_id.py

@@ -0,0 +1,24 @@
+# Generated manually: ActiveSite.source_site_id from city Site.site_id
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("sync", "0005_rename_registered_by_to_managed_by"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="activesite",
+            name="source_site_id",
+            field=models.SlugField(
+                blank=True,
+                db_index=True,
+                default="",
+                help_text="Farm-location Site.site_id (slug) from the registering dashboard",
+                max_length=64,
+            ),
+        ),
+    ]