fix: run init_db with panel venv python

Use venv interpreter so werkzeug is available; sync admin user from .env on init.
Set default panel username to dekun.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -14,7 +14,7 @@ ACME_EMAIL=admin@hyf2.cc
 REALITY_SERVER_NAME=www.microsoft.com
 
 # 管理面板登录（安装完成后访问 https://域名:8444）
-PANEL_USERNAME=admin
+PANEL_USERNAME=dekun
 
 # 以下由 scripts/generate-keys.sh 自动生成
 # REALITY_PRIVATE_KEY=