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/templates/admin/add_product.html

@@ -10,6 +10,21 @@
                 <h2>Add New Product</h2>
             </div>
             <div class="card-body">
+                {% if not custom_fields %}
+                <div class="alert alert-warning" role="alert">
+                    <h4 class="alert-heading"><i class="bi bi-exclamation-triangle-fill me-2"></i>No Fields Configured</h4>
+                    <p>Unable to add products. You must configure custom AR fields before adding products.</p>
+                    <hr>
+                    <p class="mb-0">
+                        <a href="/{{ tenant_id }}/ar_fields" class="btn btn-primary">
+                            <i class="bi bi-gear-fill me-1"></i>Configure AR Fields
+                        </a>
+                        <a href="/{{ tenant_id }}/" class="btn btn-secondary ms-2">
+                            <i class="bi bi-arrow-left me-1"></i>Back to Products
+                        </a>
+                    </p>
+                </div>
+                {% else %}
                 <form action="/{{ tenant_id }}/add" method="POST" enctype="multipart/form-data">
                     <div class="mb-3">
                         <label for="product_id" class="form-label">Product ID *</label>
@@ -71,6 +86,7 @@
                         <button type="submit" class="btn btn-primary">Add Product</button>
                     </div>
                 </form>
+                {% endif %}
             </div>
         </div>
     </div>