init project setup of the backend with dart and go as well as the frontend with nextjs

backend_dart/lib/infrastructure/persistence/postgres_user_repository.dart

@@ -0,0 +1,60 @@
+import 'package:backend_dart/domain/entities/user.dart';
+import 'package:backend_dart/domain/repositories/user_repository.dart';
+import 'package:postgres/postgres.dart';
+
+class PostgresUserRepository implements UserRepository {
+  final PostgreSQLConnection connection;
+
+  PostgresUserRepository(this.connection);
+
+  @override
+  Future<void> create(User user) async {
+    await connection.query(
+      'INSERT INTO users (id, name, email, password) VALUES (@id, @name, @mail, @pwd)',
+      substitutionValues: {
+        'id': user.id,
+        'name': user.name,
+        'mail': user.email,
+        'pwd': user.password,
+      },
+    );
+  }
+
+  @override
+  Future<User?> findByEmail(String email) async {
+    final results = await connection.query(
+      'SELECT id, name, email, password FROM users WHERE email = @mail',
+      substitutionValues: {'mail': email},
+    );
+
+    if (results.isNotEmpty) {
+      final row = results.first;
+      return User(
+        id: row[0],
+        name: row[1],
+        email: row[2],
+        password: row[3],
+      );
+    }
+    return null;
+  }
+
+  @override
+  Future<User?> findById(String id) async {
+    final results = await connection.query(
+      'SELECT id, name, email, password FROM users WHERE id = @id',
+      substitutionValues: {'id': id},
+    );
+
+    if (results.isNotEmpty) {
+      final row = results.first;
+      return User(
+        id: row[0],
+        name: row[1],
+        email: row[2],
+        password: row[3],
+      );
+    }
+    return null;
+  }
+}