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/definition.py

@@ -86,6 +86,7 @@ tenancy_module = ModuleDefinition(
             "merchants",
             "stores",
             "admin-users",
+            "merchant-users",
         ],
         FrontendType.STORE: [
             "team",
@@ -95,11 +96,10 @@ tenancy_module = ModuleDefinition(
     menus={
         FrontendType.ADMIN: [
             MenuSectionDefinition(
-                id="superAdmin",
-                label_key="tenancy.menu.super_admin",
-                icon="shield",
+                id="userManagement",
+                label_key="tenancy.menu.user_management",
+                icon="users",
                 order=10,
-                is_super_admin_only=True,
                 items=[
                     MenuItemDefinition(
                         id="admin-users",
@@ -108,6 +108,15 @@ tenancy_module = ModuleDefinition(
                         route="/admin/admin-users",
                         order=10,
                         is_mandatory=True,
+                        is_super_admin_only=True,
+                    ),
+                    MenuItemDefinition(
+                        id="merchant-users",
+                        label_key="tenancy.menu.merchant_users",
+                        icon="user-group",
+                        route="/admin/merchant-users",
+                        order=20,
+                        is_mandatory=True,
                     ),
                 ],
             ),