feat: implement module-driven context providers for dynamic page context

Introduces a module-driven context provider system that allows modules to
dynamically contribute template context variables without hardcoding imports.

Key changes:
- Add context_providers field to ModuleDefinition in app/modules/base.py
- Create unified get_context_for_frontend() that queries enabled modules only
- Add context providers to CMS module (PLATFORM, STOREFRONT)
- Add context providers to billing module (PLATFORM)
- Fix SQLAlchemy cross-module relationship resolution (Order, AdminMenuConfig,
  MarketplaceImportJob) by ensuring models are imported before referencing
- Document the entire system in docs/architecture/module-system.md

Benefits:
- Zero coupling: adding/removing modules requires no route handler changes
- Lazy loading: module code only imported when that module is enabled
- Per-platform customization: each platform loads only what it needs
- Graceful degradation: one failing module doesn't break entire page

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

app/modules/marketplace/models/__init__.py

@@ -14,6 +14,14 @@ Usage:
     )
 """
 
+# Import models from other modules FIRST to resolve string-based relationship references.
+# These imports are NOT re-exported, just ensure models are registered with SQLAlchemy
+# before the marketplace models are loaded.
+#
+# Relationship being resolved:
+# - LetzshopFulfillmentQueue.order -> "Order" (in orders module)
+from app.modules.orders.models import Order  # noqa: F401
+
 from app.modules.marketplace.models.marketplace_product import (
     MarketplaceProduct,
     ProductType,