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/cmd/openteam/main.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"embed"
+	"fmt"
+	"io/fs"
+	"log"
+	"opencatd-open/internal/cli"
+	"opencatd-open/internal/store"
+	"opencatd-open/pkg/config"
+	"opencatd-open/router"
+
+	"github.com/spf13/cobra"
+)
+
+//go:embed all:dist
+var web embed.FS
+
+func main() {
+	cfg, err := config.LoadConfig()
+	if err != nil {
+		panic(err)
+	}
+
+	db, err := store.InitDB(cfg)
+	if err != nil {
+		panic(err)
+	}
+	_ = db
+
+	rootCmd := &cobra.Command{
+		Use:   "openteam",
+		Short: "openteam cli",
+		Run: func(cmd *cobra.Command, args []string) {
+			router.SetRouter(cfg, db, &web)
+		},
+	}
+
+	rootCmd.AddCommand(cli.LoadCmd)
+	if err := rootCmd.Execute(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func printFilesAndDirs(fsys fs.FS, prefix string) error {
+	return fs.WalkDir(fsys, ".", func(p string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			fmt.Printf("%s[DIR] %s\n", prefix, p)
+		} else {
+			info, err := d.Info()
+			if err != nil {
+				return err
+			}
+			fmt.Printf("%s[FILE] %s (%d bytes)\n", prefix, p, info.Size())
+		}
+		return nil
+	})
+}