feat: signed releases and a host-side updater for container nodes

CI signs the image digest of each agent and probe release; the panel only
stores and forwards it. Nodes write the manifest to their data directory and
a host updater verifies the signature with a local public key, pulls by
digest, restarts the unit and rolls back when it is unhealthy. The panel can
list releases, ask a node to update and show the reported update state.

backend/src/Monitor.Backend.Api/Contracts/ApiJsonContext.cs

@@ -38,6 +38,9 @@ namespace Monitor.Backend.Api;
 [JsonSerializable(typeof(PageDto<AgentDto>))]
 [JsonSerializable(typeof(AgentPatchRequest))]
 [JsonSerializable(typeof(BuildDto))]
+[JsonSerializable(typeof(ReleaseDto))]
+[JsonSerializable(typeof(PageDto<ReleaseDto>))]
+[JsonSerializable(typeof(PublishReleaseRequest))]
 [JsonSerializable(typeof(AboutDto))]
 [JsonSerializable(typeof(NodeBuildDto))]
 [JsonSerializable(typeof(BuildInfoDto))]