feat: implement metrics provider pattern for modular dashboard statistics

This commit introduces a protocol-based metrics architecture that allows
each module to provide its own statistics for dashboards without creating
cross-module dependencies.

Key changes:
- Add MetricsProviderProtocol and MetricValue dataclass in contracts module
- Add StatsAggregatorService in core module that discovers and aggregates
  metrics from all enabled modules
- Implement metrics providers for all modules:
  - tenancy: vendor/user counts, team members, domains
  - customers: customer counts
  - cms: pages, media files
  - catalog: products
  - inventory: stock levels
  - orders: order counts, revenue
  - marketplace: import jobs, staging products
- Update dashboard routes to use StatsAggregator instead of direct imports
- Fix VendorPlatform junction table usage (Vendor.platform_id doesn't exist)
- Add comprehensive documentation for the pattern

This architecture ensures:
- Dashboards always work (aggregator in core)
- Each module owns its metrics (no cross-module coupling)
- Optional modules are truly optional (can be removed without breaking app)
- Multi-platform vendors are properly supported via VendorPlatform table

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

app/modules/contracts/__init__.py

@@ -21,12 +21,34 @@ Usage:
                 from app.modules.cms.services import content_page_service
                 self._content = content_page_service
             return self._content
+
+Metrics Provider Pattern:
+    from app.modules.contracts.metrics import MetricsProviderProtocol, MetricValue
+
+    class OrderMetricsProvider:
+        @property
+        def metrics_category(self) -> str:
+            return "orders"
+
+        def get_vendor_metrics(self, db, vendor_id, context=None) -> list[MetricValue]:
+            return [MetricValue(key="orders.total", value=42, label="Total", category="orders")]
 """
 
 from app.modules.contracts.base import ServiceProtocol
 from app.modules.contracts.cms import ContentServiceProtocol
+from app.modules.contracts.metrics import (
+    MetricValue,
+    MetricsContext,
+    MetricsProviderProtocol,
+)
 
 __all__ = [
+    # Base protocols
     "ServiceProtocol",
+    # CMS protocols
     "ContentServiceProtocol",
+    # Metrics protocols
+    "MetricValue",
+    "MetricsContext",
+    "MetricsProviderProtocol",
 ]