系统设置迁入后台：站点地址 / OAuth 凭据 / R2 存储 / 管理员账号，保存即生效无需重启

- 生效规则统一为「后台填了用后台的，没填回落环境变量」，老部署不改 env 照常跑
- config.Resolver：短缓存解析有效配置，存储后端按配置签名热重建；后台保存主动失效
- 秘密项（client secret / bot token / R2 密钥 / 管理员密码）接口永不回显明文，
  只报「是否已配置、来自哪里」；留空保存 = 保持现值
- 管理员密码 bcrypt 入库，DB 哈希优先、显式设置的 env 密码作解锁后路；
  后台改过密码后 admin/admin 开发模式立即失效
- 设置页新增「登录与存储」标签，基础信息加站点地址；秘密项带来源提示
- 监听地址 / 数据库 / 目录 / ONE_SECRET / Passkey 仍留环境变量（启动期依赖）

backend/internal/admin/passkey.go

@@ -26,7 +26,7 @@ func (a *API) listPasskeys(w http.ResponseWriter, r *http.Request) {
 		httpx.Error(w, http.StatusMethodNotAllowed, "GET required")
 		return
 	}
-	owner, err := a.Store.EnsureOwner(a.Cfg.AdminUser)
+	owner, err := a.Store.EnsureOwner(a.cfg().AdminUser)
 	if err != nil {
 		httpx.ServerError(w, err)
 		return
@@ -44,7 +44,7 @@ func (a *API) beginPasskey(w http.ResponseWriter, r *http.Request) {
 		httpx.Error(w, http.StatusServiceUnavailable, "passkey 未启用")
 		return
 	}
-	owner, err := a.Store.EnsureOwner(a.Cfg.AdminUser)
+	owner, err := a.Store.EnsureOwner(a.cfg().AdminUser)
 	if err != nil {
 		httpx.ServerError(w, err)
 		return
@@ -84,7 +84,7 @@ func (a *API) finishPasskey(w http.ResponseWriter, r *http.Request) {
 	if name == "" {
 		name = "未命名设备"
 	}
-	owner, err := a.Store.EnsureOwner(a.Cfg.AdminUser)
+	owner, err := a.Store.EnsureOwner(a.cfg().AdminUser)
 	if err != nil {
 		httpx.ServerError(w, err)
 		return
@@ -124,7 +124,7 @@ func (a *API) deletePasskey(w http.ResponseWriter, r *http.Request) {
 		httpx.BadRequest(w, "bad passkey id")
 		return
 	}
-	owner, err := a.Store.EnsureOwner(a.Cfg.AdminUser)
+	owner, err := a.Store.EnsureOwner(a.cfg().AdminUser)
 	if err != nil {
 		httpx.ServerError(w, err)
 		return