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/service/settings_view.go

@@ -18,6 +18,13 @@ type SystemSettings struct {
 	TurnstileSecretKey           string
 	TurnstileSecretKeyConfigured bool
 
+	// LinuxDo Connect OAuth login (end-user SSO)
+	LinuxDoConnectEnabled                bool
+	LinuxDoConnectClientID               string
+	LinuxDoConnectClientSecret           string
+	LinuxDoConnectClientSecretConfigured bool
+	LinuxDoConnectRedirectURL            string
+
 	SiteName     string
 	SiteLogo     string
 	SiteSubtitle string