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/dao/token.go

@@ -0,0 +1,38 @@
+package dao
+
+import (
+	"errors"
+	"opencatd-open/internal/store"
+
+	"gorm.io/gorm"
+)
+
+type TokenDAO struct {
+	db *gorm.DB
+}
+
+func NewTokenDAO(db *gorm.DB) *TokenDAO {
+	return &TokenDAO{db: db}
+}
+
+func (d *TokenDAO) GetByKey(key string) (*store.User, error) {
+	var user store.User
+	err := d.db.Where("username = ?", key).First(&user).Error
+	if err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+func (d *TokenDAO) GetByID(id uint64) (*store.User, error) {
+	var user store.User
+	err := d.db.First(&user, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+// Placeholder to avoid compile errors - will be expanded in Phase 1
+var _ = errors.New
+var _ = gorm.ErrRecordNotFound