fleet: send log counters in heartbeats and pull the text on demand

Heartbeats carry per-level counts, the most frequent log types (a stable hash of the message) and the newest id, no text. The node keeps its last 2048 lines; moshiro.logs.fetch lets the backend pull them. The log page pulls when it is open, and the backend pulls warnings and errors once a minute after a node reports an error.

backend/src/MoShiro.Backend.Gateway/FleetGateway.cs

@@ -50,6 +50,15 @@ public sealed class FleetGateway : IAgentGateway
         await Connections.RequestAsync(
             agent, MoShiroMethods.TestCancel, new TestCancelParams { TestId = test }, MoShiroJsonContext.Default.TestCancelParams, MoShiroJsonContext.Default.TestRunResult, ct);
 
+    public async ValueTask<Result<LogsFetchResult>> FetchLogsAsync(NodeId node, LogsFetchParams request, CancellationToken ct)
+    {
+        Result<LogsFetchResult> result = await Connections.RequestAsync(
+            node, MoShiroMethods.LogsFetch, request, MoShiroJsonContext.Default.LogsFetchParams, MoShiroJsonContext.Default.LogsFetchResult, ct);
+        return result.TryGet(out LogsFetchResult? page, out FleetError? error)
+            ? Result<LogsFetchResult>.Ok(page)
+            : Result<LogsFetchResult>.Fail(error.Code == FleetErrors.NodeOffline ? MoShiroErrors.AgentOffline : error.Code, error.Message);
+    }
+
     public async ValueTask<Result<bool>> DisconnectDialerAsync(NodeId listener, NodeId dialer, CancellationToken ct)
     {
         Result<TestRunResult> result = await Connections.RequestAsync(