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/usage.go

@@ -0,0 +1,22 @@
+package service
+
+import (
+	"context"
+	"opencatd-open/pkg/config"
+
+	"gorm.io/gorm"
+)
+
+type UsageService struct {
+	Ctx context.Context
+	Cfg *config.Config
+	DB  *gorm.DB
+}
+
+func NewUsageService(ctx context.Context, cfg *config.Config, db *gorm.DB) *UsageService {
+	return &UsageService{
+		Ctx: ctx,
+		Cfg: cfg,
+		DB:  db,
+	}
+}