refactor(auth): 将 Linux DO OAuth 配置迁移到系统设置

- 将 LinuxDo Connect 配置从环境变量迁移到数据库持久化
- 在管理后台系统设置中添加 LinuxDo OAuth 配置项
- 简化部署流程，无需修改 docker-compose.override.yml

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/i18n/locales/en.ts

@@ -1766,6 +1766,24 @@ export default {
         cloudflareDashboard: 'Cloudflare Dashboard',
         secretKeyHint: 'Server-side verification key (keep this secret)',
         secretKeyConfiguredHint: 'Secret key configured. Leave empty to keep the current value.'      },
+      linuxdo: {
+        title: 'LinuxDo Connect Login',
+        description: 'Configure LinuxDo Connect OAuth for Sub2API end-user login',
+        enable: 'Enable LinuxDo Login',
+        enableHint: 'Show LinuxDo login on the login/register pages',
+        clientId: 'Client ID',
+        clientIdPlaceholder: 'e.g., hprJ5pC3...',
+        clientIdHint: 'Get this from Connect.Linux.Do',
+        clientSecret: 'Client Secret',
+        clientSecretPlaceholder: '********',
+        clientSecretHint: 'Used by backend to exchange tokens (keep it secret)',
+        clientSecretConfiguredPlaceholder: '********',
+        clientSecretConfiguredHint: 'Secret configured. Leave empty to keep the current value.',
+        redirectUrl: 'Redirect URL',
+        redirectUrlPlaceholder: 'https://your-domain.com/api/v1/auth/oauth/linuxdo/callback',
+        redirectUrlHint:
+          'Must match the redirect URL configured in Connect.Linux.Do (must be an absolute http(s) URL)'
+      },
       defaults: {
         title: 'Default User Settings',
         description: 'Default values for new users',