feat(tenancy): add merchant-level domain with store override

Merchants can now register domains (e.g., myloyaltyprogram.lu) that all
their stores inherit. Individual stores can override with their own custom
domain. Resolution priority: StoreDomain > MerchantDomain > subdomain.

- Add MerchantDomain model, schema, service, and admin API endpoints
- Add merchant domain fallback in platform and store context middleware
- Add Merchant.primary_domain and Store.effective_domain properties
- Add Alembic migration for merchant_domains table
- Update loyalty user journey docs with subscription & domain setup flow
- Add unit tests (50 passing) and integration tests (15 passing)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/tenancy/models/store.py

@@ -329,6 +329,22 @@ class Store(Base, TimestampMixin):
                 return domain.domain  # Return the domain if it's primary and active
         return None
 
+    @property
+    def effective_domain(self) -> str | None:
+        """
+        Get effective domain: store override > merchant domain > subdomain fallback.
+
+        Domain Resolution Priority:
+        1. Store-specific custom domain (StoreDomain) -> highest priority
+        2. Merchant domain (MerchantDomain) -> inherited default
+        3. Store subdomain ({store.subdomain}.{platform_domain}) -> fallback
+        """
+        if self.primary_domain:
+            return self.primary_domain
+        if self.merchant and self.merchant.primary_domain:
+            return self.merchant.primary_domain
+        return f"{self.subdomain}.{settings.platform_domain}"
+
     @property
     def all_domains(self):
         """Get all active domains (subdomain + custom domains)."""