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/cms/definition.py

@@ -12,9 +12,100 @@ This is a self-contained module with:
 - Templates: app.modules.cms.templates (namespaced as cms/)
 """
 
+import logging
+from typing import Any
+
 from app.modules.base import MenuItemDefinition, MenuSectionDefinition, ModuleDefinition, PermissionDefinition
 from app.modules.enums import FrontendType
 
+logger = logging.getLogger(__name__)
+
+
+# =============================================================================
+# Context Providers
+# =============================================================================
+
+
+def _get_platform_context(request: Any, db: Any, platform: Any) -> dict[str, Any]:
+    """
+    Provide CMS context for platform/marketing pages.
+
+    Returns header and footer navigation pages for the marketing site.
+    """
+    from app.modules.cms.services import content_page_service
+
+    platform_id = platform.id if platform else 1
+
+    header_pages = []
+    footer_pages = []
+
+    try:
+        header_pages = content_page_service.list_platform_pages(
+            db, platform_id=platform_id, header_only=True, include_unpublished=False
+        )
+        footer_pages = content_page_service.list_platform_pages(
+            db, platform_id=platform_id, footer_only=True, include_unpublished=False
+        )
+        logger.debug(
+            f"[CMS] Platform context: {len(header_pages)} header, {len(footer_pages)} footer pages"
+        )
+    except Exception as e:
+        logger.warning(f"[CMS] Failed to load platform navigation pages: {e}")
+
+    return {
+        "header_pages": header_pages,
+        "footer_pages": footer_pages,
+        "legal_pages": [],  # TODO: Add legal pages support if needed
+    }
+
+
+def _get_storefront_context(request: Any, db: Any, platform: Any) -> dict[str, Any]:
+    """
+    Provide CMS context for storefront (customer shop) pages.
+
+    Returns header and footer navigation pages for the vendor's shop.
+    """
+    from app.modules.cms.services import content_page_service
+
+    vendor = getattr(request.state, "vendor", None)
+    platform_id = platform.id if platform else 1
+
+    header_pages = []
+    footer_pages = []
+
+    if vendor:
+        try:
+            header_pages = content_page_service.list_pages_for_vendor(
+                db,
+                platform_id=platform_id,
+                vendor_id=vendor.id,
+                header_only=True,
+                include_unpublished=False,
+            )
+            footer_pages = content_page_service.list_pages_for_vendor(
+                db,
+                platform_id=platform_id,
+                vendor_id=vendor.id,
+                footer_only=True,
+                include_unpublished=False,
+            )
+            logger.debug(
+                f"[CMS] Storefront context for vendor {vendor.id}: "
+                f"{len(header_pages)} header, {len(footer_pages)} footer pages"
+            )
+        except Exception as e:
+            logger.warning(f"[CMS] Failed to load storefront navigation pages: {e}")
+
+    return {
+        "header_pages": header_pages,
+        "footer_pages": footer_pages,
+    }
+
+
+# =============================================================================
+# Router Lazy Imports
+# =============================================================================
+
 
 def _get_admin_router():
     """Lazy import of admin router to avoid circular imports."""
@@ -139,6 +230,11 @@ cms_module = ModuleDefinition(
         ],
     },
     is_core=True,  # CMS is a core module - content management is fundamental
+    # Context providers for dynamic page context
+    context_providers={
+        FrontendType.PLATFORM: _get_platform_context,
+        FrontendType.STOREFRONT: _get_storefront_context,
+    },
     # Self-contained module configuration
     is_self_contained=True,
     services_path="app.modules.cms.services",