feat: consolidate media service, add merchant users page, fix metrics overlap

- Merge ImageService into MediaService with WebP variant generation,
  DB-backed storage stats, and module-driven media usage discovery
  via new MediaUsageProviderProtocol
- Add merchant users admin page with scoped user listing, stats
  endpoint, template, JS, and i18n strings (de/en/fr/lb)
- Fix merchant user metrics so Owners and Team Members are mutually
  exclusive (filter team_members on user_type="member" and exclude
  owner IDs) ensuring stat cards add up correctly
- Update billing and monitoring services to use media_service
- Update subscription-billing and feature-gating docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/catalog/definition.py

@@ -43,6 +43,13 @@ def _get_feature_provider():
     return catalog_feature_provider
 
 
+def _get_media_usage_provider():
+    """Lazy import of media usage provider to avoid circular imports."""
+    from app.modules.catalog.services.product_media_service import product_media_service
+
+    return product_media_service
+
+
 # Catalog module definition
 catalog_module = ModuleDefinition(
     code="catalog",
@@ -122,6 +129,7 @@ catalog_module = ModuleDefinition(
     # Metrics provider for dashboard statistics
     metrics_provider=_get_metrics_provider,
     feature_provider=_get_feature_provider,
+    media_usage_provider=_get_media_usage_provider,
 )