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/errors.py

@@ -0,0 +1,10 @@
+"""storefronts domain errors."""
+from __future__ import annotations
+
+
+class StorefrontsError(Exception):
+    """Base for storefronts-domain errors."""
+
+
+class AlreadyOwnsStorefront(StorefrontsError):
+    """INV-4: the account already has its one storefront (PUC-7)."""