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/Filament/User/Resources/ProductResource.php

@@ -41,6 +41,11 @@ class ProductResource extends Resource {
                 Forms\Components\TextInput::make('title')
                     ->label('Name')
                     ->required(),
+                Forms\Components\TextInput::make('lot_prefix')
+                    ->label('Chargen-Präfix')
+                    ->placeholder('z.B. 112')
+                    ->unique(ignoreRecord: true)
+                    ->maxLength(20),
                 Placeholder::make('shopify_product_id')
                     ->label('Shopify ID')
                     ->content(fn ($record) => $record?->shopify_product_id