feat: add alot

lib/database/stock_queries.dart

@@ -0,0 +1,70 @@
+import 'package:drift/drift.dart';
+
+import 'app_database.dart';
+
+class InsufficientStockException implements Exception {
+  const InsufficientStockException();
+
+  @override
+  String toString() => 'Not enough stock';
+}
+
+class ProductNotFoundException implements Exception {
+  final String productId;
+
+  const ProductNotFoundException(this.productId);
+
+  @override
+  String toString() => 'Product not found: $productId';
+}
+
+/// Atomic stock updates shared by the product and tab services, so they can
+/// run inside either one's transaction.
+extension StockQueries on AppDatabase {
+  Future<void> decreaseStock(String productId, int amount) async {
+    _validateStockAmount(amount);
+
+    final updatedRows =
+        await (update(products)..where(
+              (row) =>
+                  row.id.equals(productId) &
+                  row.stockQuantity.isBiggerOrEqualValue(amount),
+            ))
+            .write(
+              ProductsCompanion.custom(
+                stockQuantity: products.stockQuantity - Variable(amount),
+              ),
+            );
+
+    if (updatedRows == 1) return;
+
+    final product = await (select(
+      products,
+    )..where((row) => row.id.equals(productId))).getSingleOrNull();
+
+    if (product == null) throw ProductNotFoundException(productId);
+
+    throw const InsufficientStockException();
+  }
+
+  Future<void> increaseStock(String productId, int amount) async {
+    _validateStockAmount(amount);
+
+    final updatedRows =
+        await (update(
+          products,
+        )..where((row) => row.id.equals(productId))).write(
+          ProductsCompanion.custom(
+            stockQuantity: products.stockQuantity + Variable(amount),
+          ),
+        );
+
+    if (updatedRows != 1) throw ProductNotFoundException(productId);
+  }
+
+  void _validateStockAmount(int amount) {
+    if (amount <= 0) {
+      throw ArgumentError.value(amount, 'amount', 'Must be greater than zero');
+    }
+  }
+}