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/apikey.go

@@ -0,0 +1,68 @@
+package dao
+
+import (
+	"errors"
+	"opencatd-open/internal/store"
+
+	"gorm.io/gorm"
+)
+
+type ApiKeyDAO struct {
+	db *gorm.DB
+}
+
+func NewApiKeyDAO(db *gorm.DB) *ApiKeyDAO {
+	return &ApiKeyDAO{db: db}
+}
+
+func (d *ApiKeyDAO) Create(apiKey *store.APIKey) error {
+	if apiKey == nil {
+		return errors.New("apiKey is nil")
+	}
+	return d.db.Create(apiKey).Error
+}
+
+func (d *ApiKeyDAO) GetByID(id uint64) (*store.APIKey, error) {
+	var apiKey store.APIKey
+	err := d.db.First(&apiKey, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &apiKey, nil
+}
+
+func (d *ApiKeyDAO) GetByHash(keyHash string) (*store.APIKey, error) {
+	var apiKey store.APIKey
+	err := d.db.Where("key_hash = ? AND status = ?", keyHash, store.KeyStatusActive).First(&apiKey).Error
+	if err != nil {
+		return nil, err
+	}
+	return &apiKey, nil
+}
+
+func (d *ApiKeyDAO) ListByUserID(userID uint64, limit, offset int) ([]*store.APIKey, int64, error) {
+	var apiKeys []*store.APIKey
+	var total int64
+	db := d.db.Where("user_id = ?", userID)
+	db.Model(&store.APIKey{}).Count(&total)
+	err := db.Limit(limit).Offset(offset).Order("created_at DESC").Find(&apiKeys).Error
+	return apiKeys, total, err
+}
+
+func (d *ApiKeyDAO) Update(apiKey *store.APIKey) error {
+	if apiKey == nil {
+		return errors.New("apiKey is nil")
+	}
+	return d.db.Save(apiKey).Error
+}
+
+func (d *ApiKeyDAO) Delete(id uint64) error {
+	return d.db.Delete(&store.APIKey{}, id).Error
+}
+
+func (d *ApiKeyDAO) BatchDelete(ids []uint64) error {
+	if len(ids) == 0 {
+		return errors.New("ids is empty")
+	}
+	return d.db.Delete(&store.APIKey{}, ids).Error
+}