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/api/admin/settings.ts

@@ -34,6 +34,11 @@ export interface SystemSettings {
   turnstile_enabled: boolean
   turnstile_site_key: string
   turnstile_secret_key_configured: boolean
+  // LinuxDo Connect OAuth login (end-user SSO)
+  linuxdo_connect_enabled: boolean
+  linuxdo_connect_client_id: string
+  linuxdo_connect_client_secret_configured: boolean
+  linuxdo_connect_redirect_url: string
   // Identity patch configuration (Claude -> Gemini)
   enable_identity_patch: boolean
   identity_patch_prompt: string
@@ -60,6 +65,10 @@ export interface UpdateSettingsRequest {
   turnstile_enabled?: boolean
   turnstile_site_key?: string
   turnstile_secret_key?: string
+  linuxdo_connect_enabled?: boolean
+  linuxdo_connect_client_id?: string
+  linuxdo_connect_client_secret?: string
+  linuxdo_connect_redirect_url?: string
   enable_identity_patch?: boolean
   identity_patch_prompt?: string
 }