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/contracts/__init__.py

@@ -50,7 +50,7 @@ from app.modules.contracts.audit import (
     AuditProviderProtocol,
 )
 from app.modules.contracts.base import ServiceProtocol
-from app.modules.contracts.cms import ContentServiceProtocol
+from app.modules.contracts.cms import ContentServiceProtocol, MediaUsageProviderProtocol
 from app.modules.contracts.features import (
     FeatureDeclaration,
     FeatureProviderProtocol,
@@ -78,6 +78,7 @@ __all__ = [
     "ServiceProtocol",
     # CMS protocols
     "ContentServiceProtocol",
+    "MediaUsageProviderProtocol",
     # Audit protocols
     "AuditEvent",
     "AuditProviderProtocol",