feat: implement modular platform architecture (Phase 1)

Add module system for enabling/disabling feature bundles per platform.

Module System:
- ModuleDefinition dataclass for defining modules
- 12 modules: core, platform-admin, billing, inventory, orders,
  marketplace, customers, cms, analytics, messaging, dev-tools, monitoring
- Core modules (core, platform-admin) cannot be disabled
- Module dependencies (e.g., marketplace requires inventory)

MenuService Integration:
- Menu items filtered by module enablement
- MenuItemConfig includes is_module_enabled and module_code fields
- Module-disabled items hidden from sidebar

Platform Configuration:
- BasePlatformConfig.enabled_modules property
- OMS: all modules enabled (full commerce)
- Loyalty: focused subset (no billing/inventory/orders/marketplace)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/platforms/oms/config.py

@@ -3,6 +3,15 @@
 OMS Platform Configuration
 
 Configuration for the Order Management System platform.
+
+OMS is a full-featured order management system with:
+- Inventory and product management
+- Order processing and fulfillment
+- Letzshop marketplace integration
+- Customer management
+- Billing and subscriptions
+- Content management
+- Analytics and reporting
 """
 
 from app.platforms.shared.base_platform import BasePlatformConfig
@@ -34,6 +43,32 @@ class OMSPlatformConfig(BasePlatformConfig):
             "customer_view",
         ]
 
+    @property
+    def enabled_modules(self) -> list[str]:
+        """
+        OMS enables all major commerce modules.
+
+        Core modules (core, platform-admin) are always included.
+        """
+        return [
+            # Core modules (always enabled, listed for clarity)
+            "core",
+            "platform-admin",
+            # Commerce modules
+            "billing",
+            "inventory",
+            "orders",
+            "marketplace",
+            "customers",
+            # Content & communication
+            "cms",
+            "analytics",
+            "messaging",
+            # Internal tools
+            "dev-tools",
+            "monitoring",
+        ]
+
     @property
     def vendor_default_page_slugs(self) -> list[str]:
         """Default pages for OMS vendor storefronts."""