refactor: complete backend rewrite for multi-protocol proxy

Major rewrite of the Go backend to support:
- Three API format imports: openai, anthropic, compatible
- Three protocol conversions: Chat Completions, Responses, Messages
- Hub-and-spoke architecture with Chat as intermediate format

Deleted:
- opencat.go (old entry)
- store/, team/, pkg/team/, pkg/store/ (old data layer)
- internal/model/, internal/consts/ (old types)
- internal/service/team/, internal/controller/team/ (old handlers)
- llm/ (removed LLM client library, pure proxy mode)
- dist/, assets/ (old build artifacts)

Added:
- internal/store/ — 9 GORM models + multi-DB support
- internal/pkg/ — crypto (AES-GCM), apikey, jwt, ratelimit, resp, tokenizer
- internal/channel/ — channel selection, weighted LB, health checks
- internal/proxy/convert/ — 6 protocol conversion functions + SSE streaming
- internal/proxy/ — gateway with request dispatch and upstream selection
- internal/usage/ — async usage recorder with batch writes
- internal/api/ — management API (auth, users, keys, channels, models)
- Makefile for build/test/deploy

Fixed API to match frontend expectations:
- Login response wraps token in { data: { token } }
- GET /api/profile route added
- Profile response wraps user in { code, data }
- Role returned as number (10=admin, 1=user)

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)
+}