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>

docs/architecture/module-system.md

@@ -242,6 +242,7 @@ analytics_module = ModuleDefinition(
 | `is_core` | `bool` | Cannot be disabled if True |
 | `is_internal` | `bool` | Admin-only if True |
 | `is_self_contained` | `bool` | Uses self-contained structure |
+| `metrics_provider` | `Callable` | Factory function returning MetricsProviderProtocol (see [Metrics Provider Pattern](metrics-provider-pattern.md)) |
 
 ## Route Auto-Discovery
 
@@ -1260,6 +1261,7 @@ python scripts/validate_architecture.py
 ## Related Documentation
 
 - [Creating Modules](../development/creating-modules.md) - Step-by-step guide
+- [Metrics Provider Pattern](metrics-provider-pattern.md) - Dashboard statistics architecture
 - [Menu Management](menu-management.md) - Sidebar configuration
 - [Observability](observability.md) - Health checks integration
 - [Feature Gating](../implementation/feature-gating-system.md) - Tier-based access