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/token.go

@@ -0,0 +1,29 @@
+package service
+
+import (
+	"context"
+	"opencatd-open/internal/dao"
+	"opencatd-open/internal/store"
+
+	"gorm.io/gorm"
+)
+
+type TokenServiceImpl struct {
+	db        *gorm.DB
+	tokenRepo *dao.TokenDAO
+}
+
+func NewTokenService(db *gorm.DB, tokenRepo *dao.TokenDAO) *TokenServiceImpl {
+	return &TokenServiceImpl{
+		db:        db,
+		tokenRepo: tokenRepo,
+	}
+}
+
+func (t *TokenServiceImpl) GetByKey(ctx context.Context, key string) (*store.User, error) {
+	return t.tokenRepo.GetByKey(key)
+}
+
+func (t *TokenServiceImpl) GetByID(ctx context.Context, id uint64) (*store.User, error) {
+	return t.tokenRepo.GetByID(id)
+}