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/shared/base_platform.py

@@ -16,6 +16,11 @@ class BasePlatformConfig(ABC):
 
     Each platform should create a config.py that extends this class
     and provides platform-specific settings.
+
+    Module Configuration:
+    - enabled_modules: List of module codes to enable for this platform
+    - Core modules (core, platform-admin) are always enabled
+    - If not specified, all modules are enabled (backwards compatibility)
     """
 
     @property
@@ -59,6 +64,33 @@ class BasePlatformConfig(ABC):
         """List of feature codes enabled for this platform."""
         return []
 
+    @property
+    def enabled_modules(self) -> list[str]:
+        """
+        List of module codes enabled for this platform.
+
+        Core modules (core, platform-admin) are always enabled regardless.
+        Override in subclass to customize which modules are available.
+
+        Available modules:
+        - core (always enabled): Dashboard, settings, profile
+        - platform-admin (always enabled): Companies, vendors, admin users
+        - billing: Subscription tiers, billing history
+        - inventory: Stock management, products
+        - orders: Order processing, fulfillment
+        - marketplace: Letzshop integration
+        - customers: Customer management
+        - cms: Content pages, media library
+        - analytics: Reports, dashboard analytics
+        - messaging: Messages, notifications
+        - dev-tools: Component library (internal)
+        - monitoring: Logs, background tasks (internal)
+
+        Returns:
+            List of module codes. Empty list means all modules enabled.
+        """
+        return []  # Empty = all modules enabled (backwards compatibility)
+
     @property
     def marketing_page_slugs(self) -> list[str]:
         """