fix(billing): 异步任务退款时同步减少 used_quota (#6795)

* fix(billing): 异步任务退款时同步减少 used_quota
退款时仅恢复了 quota（剩余额度），但未同步减少 used_quota（已用额度），
导致"总额度"（quota + used_quota）随退款次数持续虚增，超出用户实际充值金额。

修复三处退款路径：
- RefundTaskQuota：任务失败完整退款
- RecalculateTaskQuota：差额结算退款分支
- controller/midjourney.go：Midjourney 任务失败退款

新增 model.UpdateUserUsedQuota 公开函数，仅调整 used_quota 不影响 request_count。

* fix(billing): 任务退款时同步扣减渠道 used_quota

* fix(billing): complete async task refund accounting

* style(model): group internal Midjourney fields

---------

Co-authored-by: CaIon <i@caion.me>

model/user_update_test.go

@@ -89,6 +89,61 @@ func TestUserUpdateDoesNotOverwriteConcurrentAccountingOrTokenChanges(t *testing
 	assert.Equal(t, "rotated-token", got.GetAccessToken())
 }
 
+func TestUsageAccountingSupportsSignedDirectAndBatchDeltas(t *testing.T) {
+	setupUserUpdateTestState(t)
+	resetBatchUpdateTestState(t)
+
+	user := User{
+		Id:           10,
+		Username:     "usage-adjustment-user",
+		Password:     "password",
+		Status:       common.UserStatusEnabled,
+		UsedQuota:    1000,
+		RequestCount: 3,
+	}
+	channel := Channel{
+		Id:        10,
+		Name:      "usage-adjustment-channel",
+		Key:       "sk-test",
+		Status:    common.ChannelStatusEnabled,
+		UsedQuota: 1000,
+	}
+	require.NoError(t, DB.Create(&user).Error)
+	require.NoError(t, DB.Create(&channel).Error)
+
+	UpdateUserUsedQuota(user.Id, -200)
+	UpdateUserUsedQuota(user.Id, 50)
+	UpdateChannelUsedQuota(channel.Id, -200)
+	UpdateChannelUsedQuota(channel.Id, 50)
+
+	var got User
+	require.NoError(t, DB.Select("used_quota", "request_count").First(&got, user.Id).Error)
+	assert.Equal(t, 850, got.UsedQuota)
+	assert.Equal(t, 3, got.RequestCount)
+	var gotChannel Channel
+	require.NoError(t, DB.Select("used_quota").First(&gotChannel, channel.Id).Error)
+	assert.Equal(t, int64(850), gotChannel.UsedQuota)
+
+	common.BatchUpdateEnabled = true
+	UpdateUserUsedQuota(user.Id, 400)
+	UpdateUserUsedQuota(user.Id, -100)
+	UpdateChannelUsedQuota(channel.Id, 400)
+	UpdateChannelUsedQuota(channel.Id, -100)
+
+	require.NoError(t, DB.Select("used_quota", "request_count").First(&got, user.Id).Error)
+	assert.Equal(t, 850, got.UsedQuota, "batch deltas must remain queued until flush")
+	assert.Equal(t, 3, got.RequestCount)
+	require.NoError(t, DB.Select("used_quota").First(&gotChannel, channel.Id).Error)
+	assert.Equal(t, int64(850), gotChannel.UsedQuota, "batch deltas must remain queued until flush")
+
+	batchUpdate()
+	require.NoError(t, DB.Select("used_quota", "request_count").First(&got, user.Id).Error)
+	assert.Equal(t, 1150, got.UsedQuota)
+	assert.Equal(t, 3, got.RequestCount)
+	require.NoError(t, DB.Select("used_quota").First(&gotChannel, channel.Id).Error)
+	assert.Equal(t, int64(1150), gotChannel.UsedQuota)
+}
+
 func TestUpdateUserAccessTokenOnlyUpdatesAccessToken(t *testing.T) {
 	setupUserUpdateTestState(t)