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/messaging/services/message_attachment_service.py

@@ -206,8 +206,8 @@ class MessageAttachmentService:
         by the static file handler or a dedicated download endpoint.
         """
         # Convert local path to URL path
-        # Assumes files are served from /static/uploads or similar
-        return f"/static/{file_path}"
+        # Files are served via the /uploads mount in main.py
+        return f"/{file_path}"
 
     def get_file_content(self, file_path: str) -> bytes | None:
         """Read file content from storage."""