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>

tests/fixtures/merchant_domain_fixtures.py

@@ -0,0 +1,97 @@
+# tests/fixtures/merchant_domain_fixtures.py
+"""
+Merchant domain test fixtures.
+
+Provides fixtures for:
+- Merchants with verified merchant domains
+- Stores with merchant domain overrides (StoreDomain)
+- MerchantDomain objects for testing
+"""
+
+import uuid
+from datetime import UTC, datetime
+
+import pytest
+
+from app.modules.tenancy.models.merchant_domain import MerchantDomain
+from app.modules.tenancy.models.store_domain import StoreDomain
+
+
+@pytest.fixture
+def test_merchant_domain(db, test_merchant):
+    """Create an unverified merchant domain."""
+    unique_id = str(uuid.uuid4())[:8]
+    domain = MerchantDomain(
+        merchant_id=test_merchant.id,
+        domain=f"merchant{unique_id}.example.com",
+        is_primary=True,
+        is_active=False,
+        is_verified=False,
+        verification_token=f"mtoken_{unique_id}",
+        ssl_status="pending",
+    )
+    db.add(domain)
+    db.commit()
+    db.refresh(domain)
+    return domain
+
+
+@pytest.fixture
+def verified_merchant_domain(db, test_merchant):
+    """Create a verified and active merchant domain."""
+    unique_id = str(uuid.uuid4())[:8]
+    domain = MerchantDomain(
+        merchant_id=test_merchant.id,
+        domain=f"verified-merchant{unique_id}.example.com",
+        is_primary=True,
+        is_active=True,
+        is_verified=True,
+        verified_at=datetime.now(UTC),
+        verification_token=f"vmtoken_{unique_id}",
+        ssl_status="active",
+    )
+    db.add(domain)
+    db.commit()
+    db.refresh(domain)
+    return domain
+
+
+@pytest.fixture
+def merchant_with_domain(db, test_merchant, test_platform):
+    """Create a merchant with a verified merchant domain linked to a platform."""
+    unique_id = str(uuid.uuid4())[:8]
+    domain = MerchantDomain(
+        merchant_id=test_merchant.id,
+        platform_id=test_platform.id,
+        domain=f"testmerchant{unique_id}.example.com",
+        is_primary=True,
+        is_active=True,
+        is_verified=True,
+        verified_at=datetime.now(UTC),
+        verification_token=f"mwd_{unique_id}",
+        ssl_status="active",
+    )
+    db.add(domain)
+    db.commit()
+    db.refresh(domain)
+    return test_merchant, domain
+
+
+@pytest.fixture
+def store_with_merchant_domain_override(db, test_store):
+    """Store that overrides the merchant domain with its own StoreDomain."""
+    unique_id = str(uuid.uuid4())[:8]
+    store_domain = StoreDomain(
+        store_id=test_store.id,
+        domain=f"storeoverride{unique_id}.example.com",
+        is_primary=True,
+        is_active=True,
+        is_verified=True,
+        verified_at=datetime.now(UTC),
+        verification_token=f"sod_{unique_id}",
+        ssl_status="active",
+    )
+    db.add(store_domain)
+    db.commit()
+    db.refresh(store_domain)
+    return test_store, store_domain