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/base.py

@@ -45,6 +45,7 @@ if TYPE_CHECKING:
     from pydantic import BaseModel
 
     from app.modules.contracts.audit import AuditProviderProtocol
+    from app.modules.contracts.cms import MediaUsageProviderProtocol
     from app.modules.contracts.features import FeatureProviderProtocol
     from app.modules.contracts.metrics import MetricsProviderProtocol
     from app.modules.contracts.widgets import DashboardWidgetProviderProtocol
@@ -477,6 +478,14 @@ class ModuleDefinition:
     # The provider will be discovered by billing's FeatureAggregator service.
     feature_provider: "Callable[[], FeatureProviderProtocol] | None" = None
 
+    # =========================================================================
+    # Media Usage Provider (Module-Driven Media Usage Tracking)
+    # =========================================================================
+    # Callable that returns a MediaUsageProviderProtocol implementation.
+    # Modules that use media files (catalog, etc.) can register a provider
+    # to report where media is being used.
+    media_usage_provider: "Callable[[], MediaUsageProviderProtocol] | None" = None
+
     # =========================================================================
     # Menu Item Methods (Legacy - uses menu_items dict of IDs)
     # =========================================================================
@@ -929,6 +938,24 @@ class ModuleDefinition:
             return None
         return self.feature_provider()
 
+    # =========================================================================
+    # Media Usage Provider Methods
+    # =========================================================================
+
+    def has_media_usage_provider(self) -> bool:
+        """Check if this module has a media usage provider."""
+        return self.media_usage_provider is not None
+
+    def get_media_usage_provider_instance(self) -> "MediaUsageProviderProtocol | None":
+        """Get the media usage provider instance for this module.
+
+        Returns:
+            MediaUsageProviderProtocol instance, or None
+        """
+        if self.media_usage_provider is None:
+            return None
+        return self.media_usage_provider()
+
     # =========================================================================
     # Magic Methods
     # =========================================================================