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_llm.go

@@ -0,0 +1,67 @@
+package middleware
+
+import (
+	"net/http"
+	"opencatd-open/internal/store"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+)
+
+func AuthLLM(db *gorm.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		authToken := c.GetHeader("Authorization")
+		if authToken == "" {
+			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{
+				"error": map[string]interface{}{
+					"message": "未提供认证信息",
+					"type":    "invalid_request_error",
+				},
+			})
+			return
+		}
+
+		// Extract API key from Bearer token
+		if len(authToken) > 7 {
+			authToken = authToken[7:]
+		}
+
+		// Find API key by prefix
+		var apiKey store.APIKey
+		if err := db.Where("key_prefix = ? AND status = ?", authToken[:8], store.KeyStatusActive).First(&apiKey).Error; err != nil {
+			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{
+				"error": map[string]interface{}{
+					"message": "无效的API密钥",
+					"type":    "invalid_request_error",
+				},
+			})
+			return
+		}
+
+		// Verify full key hash
+		keyHash := store.HashAPIKey(authToken)
+		if apiKey.KeyHash != keyHash {
+			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{
+				"error": map[string]interface{}{
+					"message": "无效的API密钥",
+					"type":    "invalid_request_error",
+				},
+			})
+			return
+		}
+
+		c.Set("api_key", &apiKey)
+		c.Set("user_id", apiKey.UserID)
+		c.Next()
+	}
+}
+
+// extractAPIKey extracts the API key from the Authorization header
+func extractAPIKey(c *gin.Context) string {
+	auth := c.GetHeader("Authorization")
+	if strings.HasPrefix(auth, "Bearer ") {
+		return auth[7:]
+	}
+	return auth
+}