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/routes/pages/admin.py

@@ -243,6 +243,30 @@ async def admin_store_theme_page(
     )
 
 
+# ============================================================================
+# MERCHANT USER MANAGEMENT ROUTES (All Admins)
+# ============================================================================
+
+
+@router.get("/merchant-users", response_class=HTMLResponse, include_in_schema=False)
+async def admin_merchant_users_list_page(
+    request: Request,
+    current_user: User = Depends(
+        require_menu_access("merchant-users", FrontendType.ADMIN)
+    ),
+    db: Session = Depends(get_db),
+):
+    """
+    Render merchant users management page.
+    Shows list of all merchant users (owners and store team members).
+    Visible to all admins.
+    """
+    return templates.TemplateResponse(
+        "tenancy/admin/merchant-users.html",
+        get_admin_context(request, db, current_user),
+    )
+
+
 # ============================================================================
 # ADMIN USER MANAGEMENT ROUTES (Super Admin Only)
 # ============================================================================