Fix foreign key cascades and add validation for missing AR fields

- Enable SQLite foreign key constraints to properly cascade deletes
- Add validation to prevent adding products when no AR fields configured
- Display helpful warnings on product pages when fields are missing
- Redirect users to AR field configuration when needed

src/app/blueprints/admin/routes.py

@@ -10,6 +10,11 @@ def add_product(tenant_id):
     """Add a new product"""
     custom_fields = ARFieldModel.get_all(tenant_id)
 
+    # Prevent adding products if no fields are configured
+    if not custom_fields:
+        flash('Cannot add products. Please configure AR fields first.', 'warning')
+        return redirect(url_for('admin.manage_ar_fields', tenant_id=tenant_id))
+
     if request.method == 'POST':
         product_id = request.form.get('product_id')