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/model.go

@@ -0,0 +1,71 @@
+package dao
+
+import (
+	"opencatd-open/internal/store"
+
+	"gorm.io/gorm"
+)
+
+type ModelDAO struct {
+	db *gorm.DB
+}
+
+func NewModelDAO(db *gorm.DB) *ModelDAO {
+	return &ModelDAO{db: db}
+}
+
+func (d *ModelDAO) Create(model *store.Model) error {
+	return d.db.Create(model).Error
+}
+
+func (d *ModelDAO) GetByID(id uint64) (*store.Model, error) {
+	var model store.Model
+	err := d.db.First(&model, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &model, nil
+}
+
+func (d *ModelDAO) GetByName(name string) (*store.Model, error) {
+	var model store.Model
+	err := d.db.Where("name = ?", name).First(&model).Error
+	if err != nil {
+		return nil, err
+	}
+	return &model, nil
+}
+
+func (d *ModelDAO) List(limit, offset int) ([]*store.Model, int64, error) {
+	var models []*store.Model
+	var total int64
+	d.db.Model(&store.Model{}).Count(&total)
+	err := d.db.Limit(limit).Offset(offset).Order("sort ASC, name ASC").Find(&models).Error
+	return models, total, err
+}
+
+func (d *ModelDAO) ListEnabled() ([]*store.Model, error) {
+	var models []*store.Model
+	err := d.db.Where("enabled = ?", true).Order("sort ASC, name ASC").Find(&models).Error
+	return models, err
+}
+
+func (d *ModelDAO) Update(model *store.Model) error {
+	return d.db.Save(model).Error
+}
+
+func (d *ModelDAO) Delete(id uint64) error {
+	return d.db.Delete(&store.Model{}, id).Error
+}
+
+// Upsert creates or updates a model by name
+func (d *ModelDAO) Upsert(model *store.Model) error {
+	return d.db.Where("name = ?", model.Name).Assign(store.Model{
+		DisplayName:    model.DisplayName,
+		InputPrice:     model.InputPrice,
+		OutputPrice:    model.OutputPrice,
+		CacheReadPrice: model.CacheReadPrice,
+		Enabled:        model.Enabled,
+		Sort:           model.Sort,
+	}).FirstOrCreate(model).Error
+}