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/handler/v1.go

@@ -41,6 +41,22 @@ func (h *V1Handler) Models(c *gin.Context) {
 	})
 }
 
+// UserBalance — GET /v1/user/balance. 返回 API Key 所属用户的账户级余额
+// （剩余 / 累计已用），与具体令牌无关。
+func (h *V1Handler) UserBalance(c *gin.Context) {
+	principal, err := h.v1.Authenticate(c.Request.Context(), c.GetHeader("Authorization"))
+	if err != nil {
+		h.writeAuthError(c, err)
+		return
+	}
+	resp, err := h.v1.UserBalance(c.Request.Context(), principal)
+	if err != nil {
+		c.JSON(http.StatusInternalServerError, gin.H{"detail": "failed to load balance"})
+		return
+	}
+	c.JSON(http.StatusOK, resp)
+}
+
 // ImageGenerations — OpenAI POST /v1/images/generations (text-to-image only).
 // Accepts exactly OpenAI's fields; size→aspect ratio and quality→resolution tier
 // are mapped server-side. Returns {created, data:[{b64_json}]}.

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.