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/middleware/auth.go

@@ -0,0 +1,53 @@
+package middleware
+
+import (
+	"net/http"
+	"opencatd-open/internal/auth"
+	"opencatd-open/internal/store"
+	"opencatd-open/internal/pkg/jwt"
+
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+)
+
+func Auth(db *gorm.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		authToken := c.GetHeader("Authorization")
+		if authToken == "" {
+			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{
+				"code":  http.StatusUnauthorized,
+				"error": "未提供认证信息",
+			})
+			return
+		}
+		if len(authToken) > 7 {
+			authToken = authToken[7:]
+		}
+		claim, err := jwt.ValidateToken(authToken, auth.GetSecretKey())
+		if err != nil {
+			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{
+				"code":  http.StatusUnauthorized,
+				"error": "无效的认证信息",
+			})
+			return
+		}
+		var user store.User
+		if err := db.First(&user, claim.UserID).Error; err != nil {
+			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{
+				"code":  http.StatusUnauthorized,
+				"error": "无效的认证信息",
+			})
+			return
+		}
+		c.Set("user", &user)
+		c.Set("user_id", claim.UserID)
+		c.Set("user_role", user.Role)
+		c.Next()
+	}
+}
+
+func CheckRole(role string) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Next()
+	}
+}