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>

tests/unit/services/test_message_attachment_service.py

@@ -338,7 +338,7 @@ class TestMessageAttachmentServiceFileOperations:
     def test_get_download_url(self, attachment_service):
         """Test download URL generation."""
         url = attachment_service.get_download_url("uploads/messages/2025/01/1/abc.pdf")
-        assert url == "/static/uploads/messages/2025/01/1/abc.pdf"
+        assert url == "/uploads/messages/2025/01/1/abc.pdf"
 
     def test_get_file_content_success(self, attachment_service):
         """Test reading file content."""