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/models.py

@@ -15,6 +15,13 @@ class ActiveSite(models.Model):
     active_site_id = models.BigAutoField(primary_key=True)
     code = models.SlugField(max_length=64, unique=True, help_text="Stable id, e.g. sukawarna")
     name = models.CharField(max_length=120)
+    source_site_id = models.SlugField(
+        max_length=64,
+        blank=True,
+        default="",
+        db_index=True,
+        help_text="Farm-location Site.site_id (slug) from the registering dashboard",
+    )
     api_base_url = models.URLField(
         max_length=255,
         blank=True,
@@ -44,6 +51,13 @@ class ActiveSite(models.Model):
     class Meta:
         db_table = "active_sites"
         ordering = ["name"]
+        constraints = [
+            models.UniqueConstraint(
+                fields=["source_site_id"],
+                condition=~models.Q(source_site_id=""),
+                name="uniq_active_site_source_site_id",
+            ),
+        ]
 
     def __str__(self) -> str:
         return f"{self.name} ({self.code})"