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/tenancy/services/vendor_team_service.py

@@ -304,7 +304,7 @@ class VendorTeamService:
 
             # Cannot remove owner
             if vendor_user.is_owner:
-                raise CannotRemoveOwnerException(vendor.vendor_code)
+                raise CannotRemoveOwnerException(user_id, vendor.id)
 
             # Soft delete - just deactivate
             vendor_user.is_active = False
@@ -354,7 +354,7 @@ class VendorTeamService:
 
             # Cannot change owner's role
             if vendor_user.is_owner:
-                raise CannotRemoveOwnerException(vendor.vendor_code)
+                raise CannotRemoveOwnerException(user_id, vendor.id)
 
             # Get or create new role
             new_role = self._get_or_create_role(