feat(api): GET /v1/user/balance 查询用户余额

users 表新增 credits_used（累计消耗积分）：扣费累加、失败退款回减（refundIfNeeded / 维护清扫改走 RefundCredits），充值/CDK/签到发放不计入。接口返回 {object,balance,used,total}，Bearer API Key 鉴权，与其他 v1 接口同一 CORS 与错误格式。站内接口文档与 README 补充端点说明。

backend/internal/http/router/router.go

@@ -49,6 +49,7 @@ func New(cfg *config.Config, auth *service.AuthService, handlers Handlers) *gin.
 	engine.GET("/health", handlers.Health.Handle)
 	engine.GET("/images/:user/:name", handlers.Images.Serve)
 	engine.GET("/v1/models", handlers.V1.Models)
+	engine.GET("/v1/user/balance", handlers.V1.UserBalance)
 	engine.POST("/v1/images/generations", handlers.V1.ImageGenerations)
 	engine.POST("/v1/images/edits", handlers.V1.ImageEdits)
 	// OpenAI Sora-style async video: create job → poll → stream content.