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/contracts/__init__.py

@@ -32,6 +32,17 @@ Metrics Provider Pattern:
 
         def get_vendor_metrics(self, db, vendor_id, context=None) -> list[MetricValue]:
             return [MetricValue(key="orders.total", value=42, label="Total", category="orders")]
+
+Widget Provider Pattern:
+    from app.modules.contracts.widgets import DashboardWidgetProviderProtocol, DashboardWidget
+
+    class OrderWidgetProvider:
+        @property
+        def widgets_category(self) -> str:
+            return "orders"
+
+        def get_vendor_widgets(self, db, vendor_id, context=None) -> list[DashboardWidget]:
+            return [DashboardWidget(key="orders.recent", widget_type="list", ...)]
 """
 
 from app.modules.contracts.base import ServiceProtocol
@@ -41,6 +52,15 @@ from app.modules.contracts.metrics import (
     MetricsContext,
     MetricsProviderProtocol,
 )
+from app.modules.contracts.widgets import (
+    BreakdownWidget,
+    DashboardWidget,
+    DashboardWidgetProviderProtocol,
+    ListWidget,
+    WidgetBreakdownItem,
+    WidgetContext,
+    WidgetListItem,
+)
 
 __all__ = [
     # Base protocols
@@ -51,4 +71,12 @@ __all__ = [
     "MetricValue",
     "MetricsContext",
     "MetricsProviderProtocol",
+    # Widget protocols
+    "WidgetContext",
+    "WidgetListItem",
+    "WidgetBreakdownItem",
+    "ListWidget",
+    "BreakdownWidget",
+    "DashboardWidget",
+    "DashboardWidgetProviderProtocol",
 ]