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/controller/init.go

@@ -0,0 +1,30 @@
+package controller
+
+import (
+	"opencatd-open/internal/service"
+	"opencatd-open/pkg/config"
+
+	"gorm.io/gorm"
+)
+
+type Api struct {
+	cfg            *config.Config
+	db             *gorm.DB
+	userService    *service.UserServiceImpl
+	tokenService   *service.TokenServiceImpl
+	keyService     *service.ApiKeyServiceImpl
+	webAuthService *service.WebAuthnService
+	usageService   *service.UsageService
+}
+
+func NewApi(cfg *config.Config, db *gorm.DB, userService *service.UserServiceImpl, tokenService *service.TokenServiceImpl, keyService *service.ApiKeyServiceImpl, webAuthService *service.WebAuthnService, usageService *service.UsageService) *Api {
+	return &Api{
+		cfg:            cfg,
+		db:             db,
+		userService:    userService,
+		tokenService:   tokenService,
+		keyService:     keyService,
+		webAuthService: webAuthService,
+		usageService:   usageService,
+	}
+}