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/api/api.go

@@ -3,6 +3,7 @@ package api
 import (
 	"net/http"
 	"opencatd-open/internal/dao"
+	"opencatd-open/internal/passkey"
 	"opencatd-open/internal/store"
 	"opencatd-open/internal/pkg/apikey"
 	"opencatd-open/internal/pkg/crypto"
@@ -23,9 +24,10 @@ type Handler struct {
 	modelDAO   *dao.ModelDAO
 	usageDAO   *dao.UsageDAO
 	dailyDAO   *dao.DailyUsageDAO
+	passkeys   *passkey.Service
 }
 
-func NewHandler(db *gorm.DB) *Handler {
+func NewHandler(db *gorm.DB, passkeys *passkey.Service) *Handler {
 	return &Handler{
 		db:         db,
 		userDAO:    dao.NewUserDAO(db),
@@ -34,6 +36,7 @@ func NewHandler(db *gorm.DB) *Handler {
 		modelDAO:   dao.NewModelDAO(db),
 		usageDAO:   dao.NewUsageDAO(db),
 		dailyDAO:   dao.NewDailyUsageDAO(db),
+		passkeys:   passkeys,
 	}
 }