refactor: restructure passkey with dedicated package

- Create internal/passkey/ package with challenge session management
- Add HTTP handler layer in internal/api/passkey.go
- Simplify Passkey model to use blob credential storage
- Register passkey routes in router
- Update frontend to match new API endpoints
- Add .env.example template

backend/internal/controller/init.go

@@ -8,23 +8,21 @@ import (
 )
 
 type Api struct {
-	cfg            *config.Config
-	db             *gorm.DB
-	userService    *service.UserServiceImpl
-	tokenService   *service.TokenServiceImpl
-	keyService     *service.ApiKeyServiceImpl
-	webAuthService *service.WebAuthnService
-	usageService   *service.UsageService
+	cfg          *config.Config
+	db           *gorm.DB
+	userService  *service.UserServiceImpl
+	tokenService *service.TokenServiceImpl
+	keyService   *service.ApiKeyServiceImpl
+	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 {
+func NewApi(cfg *config.Config, db *gorm.DB, userService *service.UserServiceImpl, tokenService *service.TokenServiceImpl, keyService *service.ApiKeyServiceImpl, usageService *service.UsageService) *Api {
 	return &Api{
-		cfg:            cfg,
-		db:             db,
-		userService:    userService,
-		tokenService:   tokenService,
-		keyService:     keyService,
-		webAuthService: webAuthService,
-		usageService:   usageService,
+		cfg:          cfg,
+		db:           db,
+		userService:  userService,
+		tokenService: tokenService,
+		keyService:   keyService,
+		usageService: usageService,
 	}
 }