feat: module toggles in POS config and Instance Manager

- Add GET/PUT /pos/api/config/modules endpoints in POS config_bp.py
- Update sidebar.js to filter nav items based on enabled modules
- Add Modules section to POS config.html with toggles for WhatsApp, Marketplace, MercadoLibre
- Add module load/save logic to POS config.js
- Preload modules in app-init.js for sidebar caching

- Add tenant module management to Instance Manager
  - get_tenant_modules / update_tenant_modules in tenant_service.py
  - GET/PUT /api/tenants/<id>/modules endpoints in tenants_bp.py
  - Add modules modal to manager index.html
  - Add module editing UI and logic to manager.js
  - Add toggle-switch CSS to manager.css

config.py

@@ -11,6 +11,9 @@ if not DB_URL:
         "Example: postgresql://user:pass@localhost/nexus_autoparts"
     )
 
+MASTER_DB_URL = os.environ.get("MASTER_DB_URL") or DB_URL
+TENANT_DB_URL_TEMPLATE = os.environ.get("TENANT_DB_URL_TEMPLATE") or DB_URL.replace("nexus_autoparts", "{db_name}")
+
 # Legacy SQLite path (used only by migration script)
 SQLITE_PATH = os.path.join(
     os.path.dirname(os.path.abspath(__file__)),