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/service/apikey.go

@@ -0,0 +1,38 @@
+package service
+
+import (
+	"context"
+	"opencatd-open/internal/dao"
+	"opencatd-open/internal/store"
+
+	"gorm.io/gorm"
+)
+
+type ApiKeyServiceImpl struct {
+	db         *gorm.DB
+	apiKeyRepo *dao.ApiKeyDAO
+}
+
+func NewApiKeyService(db *gorm.DB, apiKeyDao *dao.ApiKeyDAO) *ApiKeyServiceImpl {
+	return &ApiKeyServiceImpl{db: db, apiKeyRepo: apiKeyDao}
+}
+
+func (s *ApiKeyServiceImpl) CreateApiKey(ctx context.Context, apikey *store.APIKey) error {
+	return s.apiKeyRepo.Create(apikey)
+}
+
+func (s *ApiKeyServiceImpl) GetApiKey(ctx context.Context, id uint64) (*store.APIKey, error) {
+	return s.apiKeyRepo.GetByID(id)
+}
+
+func (s *ApiKeyServiceImpl) ListApiKey(ctx context.Context, userID uint64, limit, offset int) ([]*store.APIKey, int64, error) {
+	return s.apiKeyRepo.ListByUserID(userID, limit, offset)
+}
+
+func (s *ApiKeyServiceImpl) UpdateApiKey(ctx context.Context, apikey *store.APIKey) error {
+	return s.apiKeyRepo.Update(apikey)
+}
+
+func (s *ApiKeyServiceImpl) DeleteApiKey(ctx context.Context, id uint64) error {
+	return s.apiKeyRepo.Delete(id)
+}