fix(billing): complete billing module — fix tier change, platform support, merchant portal

- Fix admin tier change: resolve tier_code→tier_id in update_subscription(),
  delegate to billing_service.change_tier() for Stripe-connected subs
- Add platform support to admin tiers page: platform column, filter dropdown,
  platform selector in create/edit modal, platform_name in tier API response
- Filter used platforms in create subscription modal on merchant detail page
- Enrich merchant portal API responses with tier code, tier_name, platform_name
- Add eager-load of platform relationship in get_merchant_subscription()
- Remove stale store_name/store_code references from merchant templates
- Add merchant tier change endpoint (POST /change-tier) and tier selector UI
  replacing broken requestUpgrade() button
- Fix subscription detail link to use platform_id instead of sub.id

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

app/modules/loyalty/tests/integration/test_storefront_loyalty.py

@@ -0,0 +1,50 @@
+# tests/integration/api/v1/loyalty/test_storefront_loyalty.py
+"""
+Integration tests for Storefront Loyalty API endpoints.
+
+Tests cover:
+- Public endpoints (program info, self-enrollment)
+- Authenticated endpoints (card, transactions)
+
+Note: Storefront endpoints require store context from middleware.
+These tests verify endpoint behavior with mocked/simulated store context.
+"""
+
+import pytest
+
+
+@pytest.mark.integration
+@pytest.mark.api
+class TestStorefrontLoyaltyEndpoints:
+    """Tests for storefront loyalty API endpoints existence."""
+
+    def test_program_endpoint_exists(self, client):
+        """Test that program info endpoint is registered."""
+        # Without proper store context, should return 404 or error
+        response = client.get("/api/v1/storefront/loyalty/program")
+        # Endpoint exists but requires store context
+        assert response.status_code in [200, 404, 422, 500]
+
+    def test_enroll_endpoint_exists(self, client):
+        """Test that enrollment endpoint is registered."""
+        response = client.post(
+            "/api/v1/storefront/loyalty/enroll",
+            json={
+                "customer_email": "test@test.com",
+                "customer_name": "Test",
+            },
+        )
+        # Endpoint exists but requires store context
+        assert response.status_code in [200, 404, 422, 500]
+
+    def test_card_endpoint_exists(self, client):
+        """Test that card endpoint is registered."""
+        response = client.get("/api/v1/storefront/loyalty/card")
+        # Endpoint exists but requires authentication and store context
+        assert response.status_code in [401, 404, 422, 500]
+
+    def test_transactions_endpoint_exists(self, client):
+        """Test that transactions endpoint is registered."""
+        response = client.get("/api/v1/storefront/loyalty/transactions")
+        # Endpoint exists but requires authentication and store context
+        assert response.status_code in [401, 404, 422, 500]