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/modules/__init__.py

@@ -0,0 +1,47 @@
+# app/modules/__init__.py
+"""
+Modular Platform Architecture.
+
+This package provides a module system for enabling/disabling feature bundles per platform.
+
+Module Hierarchy:
+    Global (SaaS Provider)
+    └── Platform (Business Product - OMS, Loyalty, etc.)
+        └── Modules (Enabled features - Billing, Marketplace, Inventory, etc.)
+            ├── Routes (API + Page routes)
+            ├── Services (Business logic)
+            ├── Menu Items (Sidebar entries)
+            └── Templates (UI components)
+
+Modules vs Features:
+- Features: Granular capabilities (e.g., analytics_dashboard, letzshop_sync)
+  Assigned to subscription tiers, gated at API route level.
+- Modules: Cohesive feature bundles (e.g., billing, marketplace, inventory)
+  Enabled/disabled per platform, contains multiple features and menu items.
+
+Usage:
+    from app.modules import module_service
+    from app.modules.base import ModuleDefinition
+    from app.modules.registry import MODULES
+
+    # Check if module is enabled for platform
+    if module_service.is_module_enabled(platform_id, "billing"):
+        ...
+
+    # Get menu items for enabled modules
+    menu_items = module_service.get_module_menu_items(platform_id, FrontendType.ADMIN)
+
+    # Get all enabled modules for platform
+    modules = module_service.get_platform_modules(platform_id)
+"""
+
+from app.modules.base import ModuleDefinition
+from app.modules.registry import MODULES
+from app.modules.service import ModuleService, module_service
+
+__all__ = [
+    "ModuleDefinition",
+    "MODULES",
+    "ModuleService",
+    "module_service",
+]