feat: dynamic merchant sidebar with module-driven menus

Replace the hardcoded merchant sidebar with a dynamic menu system driven
by module definitions, matching the existing admin frontend pattern.
Modules declare FrontendType.MERCHANT menus in their definition.py, and
a new API endpoint unions enabled modules across all platforms the
merchant is subscribed to — so loyalty only appears when enabled.

- Add MERCHANT menu definitions to core, billing, tenancy, loyalty modules
- Extend MenuDiscoveryService with enabled_module_codes parameter
- Create GET /merchants/core/menu/render/merchant endpoint
- Update merchant Alpine.js with loadMenuConfig() and dynamic section state
- Replace hardcoded sidebar.html with x-for rendering + loading skeleton + fallback
- Add 36 unit and integration tests for menu discovery, service, and endpoint

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

app/modules/billing/definition.py

@@ -165,6 +165,10 @@ billing_module = ModuleDefinition(
             "billing",  # Store billing dashboard
             "invoices",  # Store invoice history
         ],
+        FrontendType.MERCHANT: [
+            "subscriptions",  # Merchant subscriptions
+            "invoices",  # Merchant billing history
+        ],
     },
     # New module-driven menu definitions
     menus={
@@ -199,6 +203,31 @@ billing_module = ModuleDefinition(
                 ],
             ),
         ],
+        FrontendType.MERCHANT: [
+            MenuSectionDefinition(
+                id="billing",
+                label_key="billing.menu.billing_subscriptions",
+                icon="credit-card",
+                order=50,
+                items=[
+                    MenuItemDefinition(
+                        id="subscriptions",
+                        label_key="billing.menu.subscriptions",
+                        icon="clipboard-list",
+                        route="/merchants/billing/subscriptions",
+                        order=10,
+                        is_mandatory=True,
+                    ),
+                    MenuItemDefinition(
+                        id="invoices",
+                        label_key="billing.menu.billing_history",
+                        icon="currency-euro",
+                        route="/merchants/billing/invoices",
+                        order=20,
+                    ),
+                ],
+            ),
+        ],
         FrontendType.STORE: [
             MenuSectionDefinition(
                 id="sales",