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/base.py

@@ -371,6 +371,26 @@ class ModuleDefinition:
     tasks_path: str | None = None  # Python import path, e.g., "app.modules.billing.tasks"
     scheduled_tasks: list[ScheduledTask] = field(default_factory=list)
 
+    # =========================================================================
+    # Context Providers (Module-Driven Page Context)
+    # =========================================================================
+    # Callables that provide context data for page templates per frontend type.
+    # Each provider receives (request, db, platform) and returns a dict.
+    # This enables modules to contribute context only when enabled for a platform.
+    #
+    # Example:
+    #     def get_billing_platform_context(request, db, platform):
+    #         from app.modules.billing.models import TIER_LIMITS
+    #         return {"tiers": get_tiers_data()}
+    #
+    #     billing_module = ModuleDefinition(
+    #         code="billing",
+    #         context_providers={
+    #             FrontendType.PLATFORM: get_billing_platform_context,
+    #         },
+    #     )
+    context_providers: dict[FrontendType, Callable[..., dict[str, Any]]] = field(default_factory=dict)
+
     # =========================================================================
     # Menu Item Methods (Legacy - uses menu_items dict of IDs)
     # =========================================================================
@@ -695,6 +715,46 @@ class ModuleDefinition:
         else:
             return "optional"
 
+    # =========================================================================
+    # Context Provider Methods
+    # =========================================================================
+
+    def has_context_provider(self, frontend_type: FrontendType) -> bool:
+        """Check if this module has a context provider for a frontend type."""
+        return frontend_type in self.context_providers
+
+    def get_context_contribution(
+        self,
+        frontend_type: FrontendType,
+        request: Any,
+        db: Any,
+        platform: Any,
+    ) -> dict[str, Any]:
+        """
+        Get context contribution from this module for a frontend type.
+
+        Args:
+            frontend_type: The frontend type (PLATFORM, ADMIN, VENDOR, STOREFRONT)
+            request: FastAPI Request object
+            db: Database session
+            platform: Platform object (may be None for some contexts)
+
+        Returns:
+            Dict of context variables, or empty dict if no provider
+
+        Note:
+            Exceptions are caught and logged by the caller. This method
+            may raise if the provider fails.
+        """
+        provider = self.context_providers.get(frontend_type)
+        if provider is None:
+            return {}
+        return provider(request, db, platform)
+
+    def get_supported_frontend_types(self) -> list[FrontendType]:
+        """Get list of frontend types this module provides context for."""
+        return list(self.context_providers.keys())
+
     # =========================================================================
     # Magic Methods
     # =========================================================================