refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/internal/dto/response.go

@@ -0,0 +1,28 @@
+package dto
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+type Result struct {
+	Code int    `json:"code"`
+	Msg  string `json:"msg"`
+	Data any    `json:"data,omitempty"`
+}
+
+func Success(ctx *gin.Context, data any) {
+	ctx.JSON(http.StatusOK, Result{
+		Code: 200,
+		Data: data,
+		Msg:  "success",
+	})
+}
+
+func Fail(c *gin.Context, code int, err string) {
+	c.AbortWithStatusJSON(code, gin.H{
+		"code":  code,
+		"error": err,
+	})
+}