feat: add module definition completeness validation and permissions

Add new validation rules MOD-020 to MOD-023 for module definition
completeness and standardize permissions across all modules.

Changes:
- Add MOD-020: Module definitions must have required attributes
- Add MOD-021: Modules with menus should have features
- Add MOD-022: Feature modules should have permissions
- Add MOD-023: Modules with routers should use get_*_with_routers pattern

Module permissions added:
- analytics: view, export, manage_dashboards
- billing: view_tiers, manage_tiers, view_subscriptions, manage_subscriptions, view_invoices
- cart: view, manage
- checkout: view_settings, manage_settings
- cms: view_pages, manage_pages, view_media, manage_media, manage_themes
- loyalty: view_programs, manage_programs, view_rewards, manage_rewards
- marketplace: view_integration, manage_integration, sync_products
- messaging: view_messages, send_messages, manage_templates
- payments: view_gateways, manage_gateways, view_transactions

Module improvements:
- Complete cart module with features and permissions
- Complete checkout module with features and permissions
- Add features to catalog module
- Add version to cms module
- Fix loyalty platform_router attachment
- Add path definitions to payments module
- Remove empty scheduled_tasks from dev_tools module

Documentation:
- Update module-system.md with new validation rules
- Update architecture-rules.md with MOD-020 to MOD-023

Tests:
- Add unit tests for module definition completeness
- Add tests for permission structure validation

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

app/api/v1/platform/__init__.py

@@ -0,0 +1,38 @@
+# app/api/v1/public/__init__.py
+"""
+Public API endpoints (no authentication required).
+
+Includes:
+- signup: /signup/* (multi-step signup flow - cross-cutting)
+
+Auto-discovers and aggregates public routes from self-contained modules:
+- billing: /pricing/* (subscription tiers and add-ons)
+- marketplace: /letzshop-vendors/* (vendor lookup for signup)
+- core: /language/* (language preferences)
+
+These endpoints serve the marketing homepage, pricing pages, and signup flows.
+"""
+
+from fastapi import APIRouter
+
+from app.api.v1.public import signup
+from app.modules.routes import get_public_api_routes
+
+router = APIRouter()
+
+# Cross-cutting signup flow (spans auth, vendors, billing, payments)
+router.include_router(signup.router, tags=["public-signup"])
+
+# Auto-discover public routes from modules
+for route_info in get_public_api_routes():
+    if route_info.custom_prefix:
+        router.include_router(
+            route_info.router,
+            prefix=route_info.custom_prefix,
+            tags=route_info.tags,
+        )
+    else:
+        router.include_router(
+            route_info.router,
+            tags=route_info.tags,
+        )