feat: return expiry/balance based on billing type on login

- Return expiry_at for subscription mode
- Return balance for usage-based mode
- Return is_permanent for permanent members

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/router/app/auth.go

@@ -694,10 +694,27 @@ func handleAppLogin(c *gin.Context) {
 
 	service.LogVerification(c, &appModel.ID, &user.ID, "login", "用户登录: "+req.Username, req.DeviceID, nil)
 
-	response.SuccessWithMessage(c, "登录成功", gin.H{
+	// 根据运营模式返回不同参数
+	loginData := gin.H{
 		"user_id": user.ID,
 		"token":   token,
-	})
+	}
+
+	// 永久会员返回永久标识
+	if user.Balance == -1 {
+		loginData["is_permanent"] = true
+	} else if appModel.BillingType == "subscription" {
+		// 订阅模式返回到期时间
+		if user.ExpiryAt != nil {
+			loginData["expiry_at"] = user.ExpiryAt.Format("2006-01-02 15:04:05")
+			loginData["expiry_timestamp"] = user.ExpiryAt.Unix()
+		}
+	} else if appModel.BillingType != "free" {
+		// 次数/余额模式返回余额
+		loginData["balance"] = user.Balance
+	}
+
+	response.SuccessWithMessage(c, "登录成功", loginData)
 }
 
 func checkFreePeriod(app *model.Application) bool {