feat: bootstrap admin from NAV_ADMIN_USERNAME and NAV_ADMIN_PASSWORD

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

app.py

@@ -254,12 +254,18 @@ def _group_choices():
 def _ensure_default_user() -> None:
     if User.query.count() > 0:
         return
-    u = User(username="admin")
-    u.set_password("admin123")
+    username = (os.environ.get("NAV_ADMIN_USERNAME") or "admin").strip() or "admin"
+    password = os.environ.get("NAV_ADMIN_PASSWORD") or "admin123"
+    u = User(username=username)
+    u.set_password(password)
     db.session.add(u)
     db.session.add(ServiceGroup(name="默认分组", sort_order=0))
     db.session.commit()
-    print("[nav] 首次运行：默认账号 admin  密码 admin123（仅内网使用，建议自行改库或删用户后重建）")
+    print(
+        f"[nav] 首次运行：已创建管理员「{username}」"
+        "（来自 NAV_ADMIN_USERNAME / NAV_ADMIN_PASSWORD，未设置则为 admin / admin123）。"
+        "生产环境请尽快改为强密码。"
+    )
 
 
 app = create_app()