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/domain_constants.go

@@ -105,6 +105,12 @@ const (
 	// Request identity patch (Claude -> Gemini systemInstruction injection)
 	SettingKeyEnableIdentityPatch = "enable_identity_patch"
 	SettingKeyIdentityPatchPrompt = "identity_patch_prompt"
+
+	// LinuxDo Connect OAuth login (end-user SSO)
+	SettingKeyLinuxDoConnectEnabled      = "linuxdo_connect_enabled"
+	SettingKeyLinuxDoConnectClientID     = "linuxdo_connect_client_id"
+	SettingKeyLinuxDoConnectClientSecret = "linuxdo_connect_client_secret"
+	SettingKeyLinuxDoConnectRedirectURL  = "linuxdo_connect_redirect_url"
 )
 
 // AdminAPIKeyPrefix is the prefix for admin API keys (distinct from user "sk-" keys).