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/monitoring/services/platform_health_service.py

@@ -16,7 +16,7 @@ import psutil
 from sqlalchemy import func, text
 from sqlalchemy.orm import Session
 
-from app.modules.core.services.image_service import image_service
+from app.modules.cms.services.media_service import media_service
 from app.modules.inventory.models import Inventory
 from app.modules.orders.models import Order
 from app.modules.catalog.models import Product
@@ -108,9 +108,9 @@ class PlatformHealthService:
             "inventory_count": inventory_count,
         }
 
-    def get_image_storage_metrics(self) -> dict:
+    def get_image_storage_metrics(self, db: Session) -> dict:
         """Get image storage statistics."""
-        stats = image_service.get_storage_stats()
+        stats = media_service.get_storage_stats(db)
         return {
             "total_files": stats["total_files"],
             "total_size_mb": stats["total_size_mb"],
@@ -134,7 +134,7 @@ class PlatformHealthService:
         products_by_store = {name or "Unknown": count for name, count in store_counts}
 
         # Image storage
-        image_stats = image_service.get_storage_stats()
+        image_stats = media_service.get_storage_stats(db)
 
         # Database size
         db_size = self._get_database_size(db)
@@ -282,7 +282,7 @@ class PlatformHealthService:
         database = self.get_database_metrics(db)
 
         # Image storage metrics
-        image_storage = self.get_image_storage_metrics()
+        image_storage = self.get_image_storage_metrics(db)
 
         # Subscription capacity
         subscription_capacity = self.get_subscription_capacity(db)