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/user.go

@@ -0,0 +1,48 @@
+package service
+
+import (
+	"context"
+	"opencatd-open/internal/dao"
+	"opencatd-open/internal/store"
+	"opencatd-open/pkg/config"
+
+	"gorm.io/gorm"
+)
+
+type UserServiceImpl struct {
+	cfg      *config.Config
+	db       *gorm.DB
+	userRepo *dao.UserDAO
+}
+
+func NewUserService(cfg *config.Config, db *gorm.DB, userRepo *dao.UserDAO) *UserServiceImpl {
+	return &UserServiceImpl{
+		cfg:      cfg,
+		db:       db,
+		userRepo: userRepo,
+	}
+}
+
+func (s *UserServiceImpl) GetByID(ctx context.Context, id uint64) (*store.User, error) {
+	return s.userRepo.GetByID(id)
+}
+
+func (s *UserServiceImpl) GetByUsername(ctx context.Context, username string) (*store.User, error) {
+	return s.userRepo.GetByUsername(username)
+}
+
+func (s *UserServiceImpl) List(ctx context.Context, limit, offset int) ([]*store.User, int64, error) {
+	return s.userRepo.List(limit, offset)
+}
+
+func (s *UserServiceImpl) Create(ctx context.Context, user *store.User) error {
+	return s.userRepo.Create(user)
+}
+
+func (s *UserServiceImpl) Update(ctx context.Context, user *store.User) error {
+	return s.userRepo.Update(user)
+}
+
+func (s *UserServiceImpl) Delete(ctx context.Context, id uint64) error {
+	return s.userRepo.Delete(id)
+}