feat: complete modular platform architecture (Phases 1-5)

Phase 1 - Vendor Router Integration:
- Wire up vendor module routers in app/api/v1/vendor/__init__.py
- Use lazy imports via __getattr__ to avoid circular dependencies

Phase 2 - Extract Remaining Modules:
- Create 6 new module directories: customers, cms, analytics, messaging,
  dev_tools, monitoring
- Each module has definition.py and route wrappers
- Update registry to import from extracted modules

Phase 3 - Database Table Migration:
- Add PlatformModule junction table for auditable module tracking
- Add migration zc2m3n4o5p6q7_add_platform_modules_table.py
- Add modules relationship to Platform model
- Update ModuleService with JSON-to-junction-table migration

Phase 4 - Module-Specific Configuration UI:
- Add /api/v1/admin/module-config/* endpoints
- Add module-config.html template and JS

Phase 5 - Integration Tests:
- Add tests/fixtures/module_fixtures.py
- Add tests/integration/api/v1/admin/test_modules.py
- Add tests/integration/api/v1/modules/test_module_access.py

Architecture fixes:
- Fix JS-003 errors: use ...data() directly in Alpine components
- Fix JS-005 warnings: add init() guards to prevent duplicate init
- Fix API-001 errors: add MenuActionResponse Pydantic model
- Add FE-008 noqa for dynamic number input in template

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

app/modules/messaging/definition.py

@@ -0,0 +1,77 @@
+# app/modules/messaging/definition.py
+"""
+Messaging module definition.
+
+Defines the messaging module including its features, menu items,
+and route configurations.
+"""
+
+from app.modules.base import ModuleDefinition
+from models.database.admin_menu_config import FrontendType
+
+
+def _get_admin_router():
+    """Lazy import of admin router to avoid circular imports."""
+    from app.modules.messaging.routes.admin import admin_router
+
+    return admin_router
+
+
+def _get_admin_notifications_router():
+    """Lazy import of admin notifications router to avoid circular imports."""
+    from app.modules.messaging.routes.admin import admin_notifications_router
+
+    return admin_notifications_router
+
+
+def _get_vendor_router():
+    """Lazy import of vendor router to avoid circular imports."""
+    from app.modules.messaging.routes.vendor import vendor_router
+
+    return vendor_router
+
+
+def _get_vendor_notifications_router():
+    """Lazy import of vendor notifications router to avoid circular imports."""
+    from app.modules.messaging.routes.vendor import vendor_notifications_router
+
+    return vendor_notifications_router
+
+
+# Messaging module definition
+messaging_module = ModuleDefinition(
+    code="messaging",
+    name="Messaging & Notifications",
+    description="Internal messages, customer communication, and notifications.",
+    features=[
+        "customer_messaging",  # Customer communication
+        "internal_messages",  # Internal team messages
+        "notification_center",  # Notification management
+    ],
+    menu_items={
+        FrontendType.ADMIN: [
+            "messages",  # Admin messages
+            "notifications",  # Admin notifications
+        ],
+        FrontendType.VENDOR: [
+            "messages",  # Vendor messages
+            "notifications",  # Vendor notifications
+        ],
+    },
+    is_core=False,
+)
+
+
+def get_messaging_module_with_routers() -> ModuleDefinition:
+    """
+    Get messaging module with routers attached.
+
+    This function attaches the routers lazily to avoid circular imports
+    during module initialization.
+    """
+    messaging_module.admin_router = _get_admin_router()
+    messaging_module.vendor_router = _get_vendor_router()
+    return messaging_module
+
+
+__all__ = ["messaging_module", "get_messaging_module_with_routers"]