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/cors.go

@@ -0,0 +1,15 @@
+package middleware
+
+import (
+	"github.com/gin-contrib/cors"
+	"github.com/gin-gonic/gin"
+)
+
+func CORS() gin.HandlerFunc {
+	config := cors.DefaultConfig()
+	config.AllowAllOrigins = true
+	config.AllowCredentials = true
+	config.AllowMethods = []string{"GET", "POST", "PUT", "DELETE", "OPTIONS"}
+	config.AllowHeaders = []string{"*"}
+	return cors.New(config)
+}