fix(usage): 使用日志事务和幂等性修复

- UsageLogRepository.Create 返回 inserted 标志
- UsageService 使用事务保证原子性
- 避免重复扣费（幂等重试场景）
- 更新依赖注入和测试

backend/internal/server/api_contract_test.go

@@ -380,7 +380,7 @@ func newContractDeps(t *testing.T) *contractDeps {
 	apiKeyService := service.NewApiKeyService(apiKeyRepo, userRepo, groupRepo, userSubRepo, apiKeyCache, cfg)
 
 	usageRepo := newStubUsageLogRepo()
-	usageService := service.NewUsageService(usageRepo, userRepo)
+	usageService := service.NewUsageService(usageRepo, userRepo, client)
 
 	settingRepo := newStubSettingRepo()
 	settingService := service.NewSettingService(settingRepo, cfg)