feat: 売上入力画面実装開始と DB API 追加\n\n- sales_screen.dart の基本実装（商品検索・合計金額表示）\n- database_helper.dart に insertSales/getSales CRUD API 追加\n- README.md の次期実装予定タスク更新

lib/services/database_helper.dart

@@ -343,6 +343,47 @@ class DatabaseHelper {
     //_estimate_snapshots テーブルも必要に応じて追加可
   }
 
+  // ========== Sales CRUD ======
+
+  Future<int> insertSales(String saleNo, String customerName, DateTime date, List<dynamic> items) async {
+    final db = await instance.database;
+
+    // 売上番号の重複チェック
+    final existing = await db.query('sales', where: 'sale_no = ?', whereArgs: [saleNo]);
+    if (existing.isNotEmpty) {
+      throw ArgumentError('売上番号「$saleNo」は既に存在します');
+    }
+
+    final itemsJson = jsonEncode(items);
+    final totalAmount = items.fold(0, (sum, item) => sum + (item['amount'] as num).toDouble());
+
+    return await db.insert('sales', {
+      'sale_no': saleNo,
+      'customer_name': customerName,
+      'date': date.toIso8601String(),
+      'total_amount': totalAmount,
+      'tax_rate': 10,
+      'items_json': itemsJson,
+      'created_at': DateTime.now().toIso8601String(),
+      'updated_at': DateTime.now().toIso8601String(),
+    });
+  }
+
+  Future<List<dynamic>> getSales() async {
+    final db = await instance.database;
+    final List<Map<String, dynamic>> maps = await db.query('sales', orderBy: 'created_at DESC');
+
+    return (maps as List).map((json) => json);
+  }
+
+  Future<dynamic?> getSales(int id) async {
+    final db = await instance.database;
+    final maps = await db.query('sales', where: 'id = ?', whereArgs: [id]);
+
+    if (maps.isEmpty) return null;
+    return maps.first;
+  }
+
   // ========== Product CRUD ======
 
   Future<int> insert(Product product) async {