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/wire/wire.go

@@ -0,0 +1,36 @@
+//go:build wireinject
+// +build wireinject
+
+package wire
+
+import (
+	"context"
+	"opencatd-open/internal/channel"
+	"opencatd-open/internal/controller/proxy"
+	"opencatd-open/internal/dao"
+	"opencatd-open/pkg/config"
+	"sync"
+
+	"github.com/google/wire"
+	"gorm.io/gorm"
+)
+
+var daoSet = wire.NewSet(
+	dao.NewUserDAO,
+	dao.NewApiKeyDAO,
+	dao.NewTokenDAO,
+	dao.NewUsageDAO,
+	dao.NewDailyUsageDAO,
+	dao.NewChannelDAO,
+	dao.NewModelDAO,
+)
+
+var channelSet = wire.NewSet(
+	channel.NewService,
+	channel.NewHealthChecker,
+)
+
+func InitProxyHandler(ctx context.Context, cfg *config.Config, db *gorm.DB, wg *sync.WaitGroup) (*proxy.Proxy, error) {
+	wire.Build(daoSet, channelSet, proxy.NewProxy)
+	return nil, nil
+}