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/catalog/definition.py

@@ -29,6 +29,13 @@ def _get_vendor_router():
     return vendor_router
 
 
+def _get_metrics_provider():
+    """Lazy import of metrics provider to avoid circular imports."""
+    from app.modules.catalog.services.catalog_metrics import catalog_metrics_provider
+
+    return catalog_metrics_provider
+
+
 # Catalog module definition
 catalog_module = ModuleDefinition(
     code="catalog",
@@ -105,6 +112,8 @@ catalog_module = ModuleDefinition(
             ),
         ],
     },
+    # Metrics provider for dashboard statistics
+    metrics_provider=_get_metrics_provider,
 )