feat(slice-3): storefronts domain — create + membership + INV-4 guard (§6.5)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/domains/storefronts/__init__.py

@@ -0,0 +1,19 @@
+"""storefronts domain — storefront entity, membership, the one-storefront guard.
+
+Owns INV-4 (one storefront per account is a service-layer rule) and INV-5 (tenant rows
+carry storefront_id). Knows nothing about identity (that is the accounts domain). Imported
+via this package surface only (§6.2).
+"""
+from __future__ import annotations
+
+from .errors import AlreadyOwnsStorefront, StorefrontsError
+from .models import Storefront
+from .service import create_storefront, storefront_for
+
+__all__ = [
+    "Storefront",
+    "StorefrontsError",
+    "AlreadyOwnsStorefront",
+    "create_storefront",
+    "storefront_for",
+]