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/tenancy/models/__init__.py

@@ -10,6 +10,17 @@ This is the canonical location for tenancy module models including:
 - Vendor domains
 """
 
+# 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 tenancy models are loaded.
+#
+# Relationship chain being resolved:
+# - Platform.admin_menu_configs -> "AdminMenuConfig" (in core module)
+# - User.marketplace_import_jobs -> "MarketplaceImportJob" (in marketplace module)
+# - Vendor.marketplace_import_jobs -> "MarketplaceImportJob" (in marketplace module)
+from app.modules.core.models import AdminMenuConfig  # noqa: F401
+from app.modules.marketplace.models.marketplace_import_job import MarketplaceImportJob  # noqa: F401
+
 from app.modules.tenancy.models.admin import (
     AdminAuditLog,
     AdminSession,