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/billing/services/capacity_forecast_service.py

@@ -47,7 +47,7 @@ class CapacityForecastService:
 
         Should be called by a daily background job.
         """
-        from app.modules.core.services.image_service import image_service
+        from app.modules.cms.services.media_service import media_service
         from app.modules.monitoring.services.platform_health_service import platform_health_service
 
         now = datetime.now(UTC)
@@ -108,7 +108,7 @@ class CapacityForecastService:
 
         # Storage metrics
         try:
-            image_stats = image_service.get_storage_stats()
+            image_stats = media_service.get_storage_stats(db)
             storage_gb = image_stats.get("total_size_gb", 0)
         except Exception:
             storage_gb = 0