phase3: add normalized domain schema, admin, services, and multistage docker build

apps/providers/services/mappings.py

@@ -0,0 +1,19 @@
+from django.contrib.contenttypes.models import ContentType
+
+from apps.providers.models import ExternalMapping
+
+
+def upsert_external_mapping(*, provider_namespace: str, external_id: str, instance, raw_payload: dict | None = None) -> ExternalMapping:
+    """Create or update a provider mapping for any internal model instance."""
+    content_type = ContentType.objects.get_for_model(instance.__class__)
+    mapping, _ = ExternalMapping.objects.update_or_create(
+        provider_namespace=provider_namespace,
+        content_type=content_type,
+        object_id=instance.pk,
+        defaults={
+            "external_id": external_id,
+            "raw_payload": raw_payload or {},
+            "is_active": True,
+        },
+    )
+    return mapping