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/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
+}