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/auth_handler.go

@@ -15,14 +15,16 @@ type AuthHandler struct {
 	cfg         *config.Config
 	authService *service.AuthService
 	userService *service.UserService
+	settingSvc  *service.SettingService
 }
 
 // NewAuthHandler creates a new AuthHandler
-func NewAuthHandler(cfg *config.Config, authService *service.AuthService, userService *service.UserService) *AuthHandler {
+func NewAuthHandler(cfg *config.Config, authService *service.AuthService, userService *service.UserService, settingService *service.SettingService) *AuthHandler {
 	return &AuthHandler{
 		cfg:         cfg,
 		authService: authService,
 		userService: userService,
+		settingSvc:  settingService,
 	}
 }