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/cms/routes/api/store_media.py

@@ -257,17 +257,29 @@ def get_media_usage(
     """
     Get where this media file is being used.
 
-    - Check products using this media
-    - Return list of usage
+    Discovers usage from all registered module providers.
     """
-    # Service will raise MediaNotFoundException if not found
-    usage = media_service.get_media_usage(
+    # Verify media belongs to store (raises MediaNotFoundException if not found)
+    media_service.get_media(
         db=db,
         store_id=current_user.token_store_id,
         media_id=media_id,
     )
 
-    return MediaUsageResponse(**usage)
+    # Discover usage from registered providers
+    from app.modules.registry import MODULES
+
+    usage = []
+    for module in MODULES.values():
+        provider = module.get_media_usage_provider_instance()
+        if provider:
+            usage.extend(provider.get_media_usage(db, media_id))
+
+    return MediaUsageResponse(
+        media_id=media_id,
+        usage=usage,
+        total_usage_count=len(usage),
+    )
 
 
 @store_media_router.post("/optimize/{media_id}", response_model=OptimizationResultResponse)