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>

backend/internal/handler/dto/settings.go

@@ -17,6 +17,11 @@ type SystemSettings struct {
 	TurnstileSiteKey             string `json:"turnstile_site_key"`
 	TurnstileSecretKeyConfigured bool   `json:"turnstile_secret_key_configured"`
 
+	LinuxDoConnectEnabled                bool   `json:"linuxdo_connect_enabled"`
+	LinuxDoConnectClientID               string `json:"linuxdo_connect_client_id"`
+	LinuxDoConnectClientSecretConfigured bool   `json:"linuxdo_connect_client_secret_configured"`
+	LinuxDoConnectRedirectURL            string `json:"linuxdo_connect_redirect_url"`
+
 	SiteName     string `json:"site_name"`
 	SiteLogo     string `json:"site_logo"`
 	SiteSubtitle string `json:"site_subtitle"`