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/marketplace/definition.py

@@ -33,6 +33,13 @@ def _get_metrics_provider():
     return marketplace_metrics_provider
 
 
+def _get_widget_provider():
+    """Lazy import of widget provider to avoid circular imports."""
+    from app.modules.marketplace.services.marketplace_widgets import marketplace_widget_provider
+
+    return marketplace_widget_provider
+
+
 # Marketplace module definition
 marketplace_module = ModuleDefinition(
     code="marketplace",
@@ -155,6 +162,8 @@ marketplace_module = ModuleDefinition(
     ],
     # Metrics provider for dashboard statistics
     metrics_provider=_get_metrics_provider,
+    # Widget provider for dashboard widgets
+    widget_provider=_get_widget_provider,
 )