docs: update documentation for platform-aware storefront routing

Update 8 documentation files to reflect new URL scheme:
- Dev: /platforms/{code}/storefront/{store_code}/
- Prod: subdomain.platform.lu/ (root path = storefront)
- Rename DEFAULT_PLATFORM_CODE to MAIN_PLATFORM_CODE
- Replace hardcoded platform_id=1 with dynamic values
- Update route examples, middleware descriptions, code samples

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/architecture/module-system.md

@@ -572,7 +572,7 @@ def _get_platform_context(request: Any, db: Any, platform: Any) -> dict[str, Any
     """Provide CMS context for platform/marketing pages."""
     from app.modules.cms.services import content_page_service
 
-    platform_id = platform.id if platform else 1
+    platform_id = platform.id if platform else None
 
     header_pages = content_page_service.list_platform_pages(
         db, platform_id=platform_id, header_only=True, include_unpublished=False