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/model.go

@@ -0,0 +1,72 @@
+package service
+
+import (
+	"context"
+	"opencatd-open/internal/dao"
+	"opencatd-open/internal/store"
+)
+
+type ModelServiceImpl struct {
+	modelDAO   *dao.ModelDAO
+	channelDAO *dao.ChannelDAO
+}
+
+func NewModelService(modelDAO *dao.ModelDAO, channelDAO *dao.ChannelDAO) *ModelServiceImpl {
+	return &ModelServiceImpl{
+		modelDAO:   modelDAO,
+		channelDAO: channelDAO,
+	}
+}
+
+func (s *ModelServiceImpl) Create(ctx context.Context, model *store.Model) error {
+	return s.modelDAO.Create(model)
+}
+
+func (s *ModelServiceImpl) GetByID(ctx context.Context, id uint64) (*store.Model, error) {
+	return s.modelDAO.GetByID(id)
+}
+
+func (s *ModelServiceImpl) GetByName(ctx context.Context, name string) (*store.Model, error) {
+	return s.modelDAO.GetByName(name)
+}
+
+func (s *ModelServiceImpl) List(ctx context.Context, limit, offset int) ([]*store.Model, int64, error) {
+	return s.modelDAO.List(limit, offset)
+}
+
+func (s *ModelServiceImpl) ListEnabled(ctx context.Context) ([]*store.Model, error) {
+	return s.modelDAO.ListEnabled()
+}
+
+func (s *ModelServiceImpl) Update(ctx context.Context, model *store.Model) error {
+	return s.modelDAO.Update(model)
+}
+
+func (s *ModelServiceImpl) Delete(ctx context.Context, id uint64) error {
+	return s.modelDAO.Delete(id)
+}
+
+func (s *ModelServiceImpl) Upsert(ctx context.Context, model *store.Model) error {
+	return s.modelDAO.Upsert(model)
+}
+
+// BindChannel binds a model to a channel
+func (s *ModelServiceImpl) BindChannel(ctx context.Context, modelID, channelID uint64, upstreamModel string, weight int) error {
+	binding := store.ChannelModelBinding{
+		ModelID:       modelID,
+		ChannelID:     channelID,
+		UpstreamModel: upstreamModel,
+		Weight:        weight,
+	}
+	return s.channelDAO.BindModels(channelID, []store.ChannelModelBinding{binding})
+}
+
+// ListChannelModels lists all models bound to a channel
+func (s *ModelServiceImpl) ListChannelModels(ctx context.Context, channelID uint64) ([]store.ChannelModelBinding, error) {
+	return s.channelDAO.GetChannelModels(channelID)
+}
+
+// ListModelChannels lists all channels for a model
+func (s *ModelServiceImpl) ListModelChannels(ctx context.Context, modelName string) ([]*store.Channel, error) {
+	return s.channelDAO.GetEnabledChannelsByModel(modelName)
+}