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/merchant.py

@@ -85,6 +85,13 @@ class Merchant(Base, TimestampMixin):
     )
     """All store brands operated by this merchant."""
 
+    merchant_domains = relationship(
+        "MerchantDomain",
+        back_populates="merchant",
+        cascade="all, delete-orphan",
+    )
+    """Custom domains registered at the merchant level (inherited by all stores)."""
+
     def __repr__(self):
         """String representation of the Merchant object."""
         return f"<Merchant(id={self.id}, name='{self.name}', stores={len(self.stores) if self.stores else 0})>"
@@ -98,6 +105,14 @@ class Merchant(Base, TimestampMixin):
         """Get the number of stores belonging to this merchant."""
         return len(self.stores) if self.stores else 0
 
+    @property
+    def primary_domain(self) -> str | None:
+        """Get the primary active and verified merchant domain."""
+        for md in self.merchant_domains:
+            if md.is_primary and md.is_active and md.is_verified:
+                return md.domain
+        return None
+
     @property
     def active_store_count(self) -> int:
         """Get the number of active stores belonging to this merchant."""