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/channel.go

@@ -0,0 +1,84 @@
+package service
+
+import (
+	"context"
+	"opencatd-open/internal/channel"
+	"opencatd-open/internal/dao"
+	"opencatd-open/internal/store"
+	"opencatd-open/internal/pkg/crypto"
+)
+
+type ChannelServiceImpl struct {
+	channelDAO  *dao.ChannelDAO
+	channelSvc  *channel.Service
+}
+
+func NewChannelService(channelDAO *dao.ChannelDAO, channelSvc *channel.Service) *ChannelServiceImpl {
+	return &ChannelServiceImpl{
+		channelDAO: channelDAO,
+		channelSvc: channelSvc,
+	}
+}
+
+func (s *ChannelServiceImpl) Create(ctx context.Context, ch *store.Channel) error {
+	return s.channelDAO.Create(ch)
+}
+
+func (s *ChannelServiceImpl) GetByID(ctx context.Context, id uint64) (*store.Channel, error) {
+	return s.channelDAO.GetByID(id)
+}
+
+func (s *ChannelServiceImpl) List(ctx context.Context, limit, offset int) ([]*store.Channel, int64, error) {
+	return s.channelDAO.List(limit, offset)
+}
+
+func (s *ChannelServiceImpl) ListEnabled(ctx context.Context) ([]*store.Channel, error) {
+	return s.channelDAO.ListEnabled()
+}
+
+func (s *ChannelServiceImpl) Update(ctx context.Context, ch *store.Channel) error {
+	return s.channelDAO.Update(ch)
+}
+
+func (s *ChannelServiceImpl) Delete(ctx context.Context, id uint64) error {
+	return s.channelDAO.Delete(id)
+}
+
+// GetAPIKey decrypts the channel's API key
+func (s *ChannelServiceImpl) GetAPIKey(ctx context.Context, channelID uint64) (string, error) {
+	ch, err := s.channelDAO.GetByID(channelID)
+	if err != nil {
+		return "", err
+	}
+	return crypto.Decrypt(ch.APIKeyEnc)
+}
+
+// SelectForModel selects the best channel for a model
+func (s *ChannelServiceImpl) SelectForModel(ctx context.Context, modelName string) (*store.Channel, error) {
+	return s.channelSvc.SelectChannel(ctx, modelName)
+}
+
+// BindModels binds models to a channel
+func (s *ChannelServiceImpl) BindModels(ctx context.Context, channelID uint64, bindings []store.ChannelModelBinding) error {
+	return s.channelDAO.BindModels(channelID, bindings)
+}
+
+// GetChannelModels returns models bound to a channel
+func (s *ChannelServiceImpl) GetChannelModels(ctx context.Context, channelID uint64) ([]store.ChannelModelBinding, error) {
+	return s.channelDAO.GetChannelModels(channelID)
+}
+
+// GetModelChannels returns channels for a model
+func (s *ChannelServiceImpl) GetModelChannels(ctx context.Context, modelName string) ([]*store.Channel, error) {
+	return s.channelDAO.GetEnabledChannelsByModel(modelName)
+}
+
+// RecordSuccess records a successful request
+func (s *ChannelServiceImpl) RecordSuccess(channelID uint64) {
+	s.channelSvc.RecordSuccess(channelID)
+}
+
+// RecordFailure records a failed request
+func (s *ChannelServiceImpl) RecordFailure(channelID uint64) {
+	s.channelSvc.RecordFailure(channelID)
+}