feat: 实现注册优惠码功能

- 支持创建/编辑/删除优惠码，设置赠送金额和使用限制
  - 注册页面实时验证优惠码并显示赠送金额
  - 支持 URL 参数自动填充 (?promo=CODE)
  - 添加优惠码验证接口速率限制
  - 使用数据库行锁防止并发超限
  - 新增后台优惠码管理页面，支持复制注册链接

backend/internal/service/auth_service_register_test.go

@@ -100,6 +100,7 @@ func newAuthService(repo *userRepoStub, settings map[string]string, emailCache E
 		emailService,
 		nil,
 		nil,
+		nil, // promoService
 	)
 }
 
@@ -131,7 +132,7 @@ func TestAuthService_Register_EmailVerifyEnabledButServiceNotConfigured(t *testi
 	}, nil)
 
 	// 应返回服务不可用错误，而不是允许绕过验证
-	_, _, err := service.RegisterWithVerification(context.Background(), "user@test.com", "password", "any-code")
+	_, _, err := service.RegisterWithVerification(context.Background(), "user@test.com", "password", "any-code", "")
 	require.ErrorIs(t, err, ErrServiceUnavailable)
 }
 
@@ -143,7 +144,7 @@ func TestAuthService_Register_EmailVerifyRequired(t *testing.T) {
 		SettingKeyEmailVerifyEnabled:  "true",
 	}, cache)
 
-	_, _, err := service.RegisterWithVerification(context.Background(), "user@test.com", "password", "")
+	_, _, err := service.RegisterWithVerification(context.Background(), "user@test.com", "password", "", "")
 	require.ErrorIs(t, err, ErrEmailVerifyRequired)
 }
 
@@ -157,7 +158,7 @@ func TestAuthService_Register_EmailVerifyInvalid(t *testing.T) {
 		SettingKeyEmailVerifyEnabled:  "true",
 	}, cache)
 
-	_, _, err := service.RegisterWithVerification(context.Background(), "user@test.com", "password", "wrong")
+	_, _, err := service.RegisterWithVerification(context.Background(), "user@test.com", "password", "wrong", "")
 	require.ErrorIs(t, err, ErrInvalidVerifyCode)
 	require.ErrorContains(t, err, "verify code")
 }