refactor: migrate templates and static files to self-contained modules

Templates Migration:
- Migrate admin templates to modules (tenancy, billing, monitoring, marketplace, etc.)
- Migrate vendor templates to modules (tenancy, billing, orders, messaging, etc.)
- Migrate storefront templates to modules (catalog, customers, orders, cart, checkout, cms)
- Migrate public templates to modules (billing, marketplace, cms)
- Keep shared templates in app/templates/ (base.html, errors/, partials/, macros/)
- Migrate letzshop partials to marketplace module

Static Files Migration:
- Migrate admin JS to modules: tenancy (23 files), core (5 files), monitoring (1 file)
- Migrate vendor JS to modules: tenancy (4 files), core (2 files)
- Migrate shared JS: vendor-selector.js to core, media-picker.js to cms
- Migrate storefront JS: storefront-layout.js to core
- Keep framework JS in static/ (api-client, utils, money, icons, log-config, lib/)
- Update all template references to use module_static paths

Naming Consistency:
- Rename static/platform/ to static/public/
- Rename app/templates/platform/ to app/templates/public/
- Update all extends and static references

Documentation:
- Update module-system.md with shared templates documentation
- Update frontend-structure.md with new module JS organization

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/integration/api/v1/admin/test_letzshop.py

@@ -178,7 +178,7 @@ class TestAdminLetzshopCredentialsAPI:
 class TestAdminLetzshopConnectionAPI:
     """Test admin Letzshop connection testing endpoints."""
 
-    @patch("app.services.letzshop.client_service.requests.Session.post")
+    @patch("app.modules.marketplace.services.letzshop.client_service.requests.Session.post")
     def test_test_vendor_connection(
         self, mock_post, client, admin_headers, test_vendor
     ):
@@ -206,7 +206,7 @@ class TestAdminLetzshopConnectionAPI:
         data = response.json()
         assert data["success"] is True
 
-    @patch("app.services.letzshop.client_service.requests.Session.post")
+    @patch("app.modules.marketplace.services.letzshop.client_service.requests.Session.post")
     def test_test_api_key_directly(self, mock_post, client, admin_headers):
         """Test any API key without associating with vendor."""
         mock_response = MagicMock()
@@ -290,7 +290,7 @@ class TestAdminLetzshopOrdersAPI:
         assert data["total"] == 1
         assert data["orders"][0]["customer_email"] == "admin-test@example.com"
 
-    @patch("app.services.letzshop.client_service.requests.Session.post")
+    @patch("app.modules.marketplace.services.letzshop.client_service.requests.Session.post")
     def test_trigger_vendor_sync(self, mock_post, client, admin_headers, test_vendor):
         """Test triggering sync for a vendor."""
         # Mock response