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

apps/providers/admin.py

@@ -0,0 +1,17 @@
+from django.contrib import admin
+
+from .models import ExternalMapping
+
+
+@admin.register(ExternalMapping)
+class ExternalMappingAdmin(admin.ModelAdmin):
+    list_display = (
+        "provider_namespace",
+        "external_id",
+        "content_type",
+        "object_id",
+        "is_active",
+        "last_seen_at",
+    )
+    list_filter = ("provider_namespace", "is_active", "content_type")
+    search_fields = ("external_id", "external_secondary_id", "provider_namespace")