feat: implement DashboardWidgetProvider pattern for modular dashboard widgets

Add protocol-based widget system following the MetricsProvider pattern:

- Create DashboardWidgetProviderProtocol in contracts/widgets.py
- Add WidgetAggregatorService in core to discover and aggregate widgets
- Implement MarketplaceWidgetProvider for recent_imports widget
- Implement TenancyWidgetProvider for recent_vendors widget
- Update admin dashboard to use widget_aggregator
- Add widget_provider field to ModuleDefinition

Architecture documentation:
- Add widget-provider-pattern.md with implementation guide
- Add cross-module-import-rules.md enforcing core/optional separation
- Update module-system.md with widget_provider and import rules

This enables modules to provide rich dashboard widgets without core modules
importing from optional modules, maintaining true module independence.

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

app/modules/base.py

@@ -45,6 +45,7 @@ if TYPE_CHECKING:
     from pydantic import BaseModel
 
     from app.modules.contracts.metrics import MetricsProviderProtocol
+    from app.modules.contracts.widgets import DashboardWidgetProviderProtocol
 
 from app.modules.enums import FrontendType
 
@@ -413,6 +414,26 @@ class ModuleDefinition:
     # The provider will be discovered by core's StatsAggregator service.
     metrics_provider: "Callable[[], MetricsProviderProtocol] | None" = None
 
+    # =========================================================================
+    # Widget Provider (Module-Driven Dashboard Widgets)
+    # =========================================================================
+    # Callable that returns a DashboardWidgetProviderProtocol implementation.
+    # Use a callable (factory function) to enable lazy loading and avoid
+    # circular imports. Each module can provide its own widgets for dashboards.
+    #
+    # Example:
+    #     def _get_widget_provider():
+    #         from app.modules.orders.services.order_widgets import order_widget_provider
+    #         return order_widget_provider
+    #
+    #     orders_module = ModuleDefinition(
+    #         code="orders",
+    #         widget_provider=_get_widget_provider,
+    #     )
+    #
+    # The provider will be discovered by core's WidgetAggregator service.
+    widget_provider: "Callable[[], DashboardWidgetProviderProtocol] | None" = None
+
     # =========================================================================
     # Menu Item Methods (Legacy - uses menu_items dict of IDs)
     # =========================================================================
@@ -799,6 +820,28 @@ class ModuleDefinition:
             return None
         return self.metrics_provider()
 
+    # =========================================================================
+    # Widget Provider Methods
+    # =========================================================================
+
+    def has_widget_provider(self) -> bool:
+        """Check if this module has a widget provider."""
+        return self.widget_provider is not None
+
+    def get_widget_provider_instance(self) -> "DashboardWidgetProviderProtocol | None":
+        """
+        Get the widget provider instance for this module.
+
+        Calls the widget_provider factory function to get the provider.
+        Returns None if no provider is configured.
+
+        Returns:
+            DashboardWidgetProviderProtocol instance, or None
+        """
+        if self.widget_provider is None:
+            return None
+        return self.widget_provider()
+
     # =========================================================================
     # Magic Methods
     # =========================================================================