fix: Make transactions atomic / AutoLogout

lib/services/product_service.dart

@@ -130,31 +130,48 @@ class DriftProductService implements ProductService {
 
   @override
   Future<void> decreaseStock(String productId, int amount) async {
-    final product = await getProductById(productId);
+    _validateStockAmount(amount);
 
-    if (product == null) {
-      throw Exception('Product not found');
-    }
-
-    if (product.stockQuantity < amount) {
-      throw const InsufficientStockException();
-    }
-
-    await updateProduct(
-      product.copyWith(stockQuantity: product.stockQuantity - amount),
+    final updatedRows = await database.customUpdate(
+      'UPDATE products '
+      'SET stock_quantity = stock_quantity - ? '
+      'WHERE id = ? AND stock_quantity >= ?',
+      variables: [
+        Variable.withInt(amount),
+        Variable.withString(productId),
+        Variable.withInt(amount),
+      ],
+      updates: {database.products},
     );
+
+    if (updatedRows == 1) return;
+
+    final product = await getProductById(productId);
+    if (product == null) throw Exception('Product not found');
+
+    throw const InsufficientStockException();
   }
 
   @override
   Future<void> increaseStock(String productId, int amount) async {
-    final product = await getProductById(productId);
+    _validateStockAmount(amount);
 
-    if (product == null) {
-      throw Exception('Product not found');
-    }
-
-    await updateProduct(
-      product.copyWith(stockQuantity: product.stockQuantity + amount),
+    final updatedRows = await database.customUpdate(
+      'UPDATE products '
+      'SET stock_quantity = stock_quantity + ? '
+      'WHERE id = ?',
+      variables: [Variable.withInt(amount), Variable.withString(productId)],
+      updates: {database.products},
     );
+
+    if (updatedRows == 1) return;
+
+    throw Exception('Product not found');
+  }
+
+  void _validateStockAmount(int amount) {
+    if (amount <= 0) {
+      throw ArgumentError.value(amount, 'amount', 'Must be greater than zero');
+    }
   }
 }