Add lot_prefix to products; compose LOT number from prefix + date suffix

Products now have a configurable lot_prefix (e.g. "112"). In the batch form
the prefix is shown read-only next to the suffix input (JJMMTT), and the full
lot_number (e.g. "112-260912") is composed automatically and stored.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/Product.php

@@ -7,7 +7,7 @@ use Illuminate\Database\Eloquent\Model;
 
 class Product extends Model
 {
-    protected $fillable = ['title', 'description', 'images', 'package_content', 'recipes_id', 'shopify_product_id'];
+    protected $fillable = ['title', 'lot_prefix', 'description', 'images', 'package_content', 'recipes_id', 'shopify_product_id'];
 
     protected $casts = [
         'images' => 'array',