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/models.py

@@ -0,0 +1,10 @@
+"""Storefront record — the §6.3 entity as the domain returns it."""
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class Storefront:
+    id: int
+    name: str