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/services/product_media_service.py

@@ -231,6 +231,26 @@ class ProductMediaService:
             "total_count": len(associations),
         }
 
+    def get_media_usage(self, db: Session, media_id: int) -> list[dict]:
+        """Implement MediaUsageProviderProtocol.
+
+        Returns list of usage records for a given media file across products.
+        """
+        rows = (
+            db.query(ProductMedia)
+            .filter(ProductMedia.media_id == media_id)
+            .all()
+        )
+        return [
+            {
+                "entity_type": "product",
+                "entity_id": r.product_id,
+                "entity_name": r.product.name if r.product else "Unknown",
+                "usage_type": r.usage_type,
+            }
+            for r in rows
+        ]
+
     def set_main_image(
         self,
         db: Session,