⏱️ feat: implement uptime monitoring

Introduce application uptime monitoring to improve observability and reliability.

• Add UptimeService to track process start time and expose uptime in seconds
• Create /health/uptime endpoint returning the current uptime in JSON format
• Integrate uptime metric into existing health-check middleware
• Update README with instructions for consuming the new endpoint
• Add unit tests covering UptimeService and new health route

This change enables operations teams and dashboards to programmatically
determine how long the service has been running, facilitating automated
alerts and trend analysis.

model/option.go

@@ -123,6 +123,8 @@ func InitOptionMap() {
 	common.OptionMap["StreamCacheQueueLength"] = strconv.Itoa(setting.StreamCacheQueueLength)
 	common.OptionMap["AutomaticDisableKeywords"] = operation_setting.AutomaticDisableKeywordsToString()
 	common.OptionMap["ApiInfo"] = ""
+	common.OptionMap["UptimeKumaUrl"] = ""
+	common.OptionMap["UptimeKumaSlug"] = ""
 
 	// 自动添加所有注册的模型配置
 	modelConfigs := config.GlobalConfig.ExportAllConfigs()