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/tenancy/locales/fr.json

@@ -1,4 +1,8 @@
 {
+  "menu": {
+    "user_management": "Gestion des utilisateurs",
+    "merchant_users": "Utilisateurs marchands"
+  },
   "team": {
     "title": "Équipe",
     "members": "Membres",