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.Application/Agents/MoShiroFleetHooks.cs

@@ -128,7 +128,7 @@ public sealed class MoShiroNodeEventSink : INodeEventSink
                 return;
             }
 
-            _nodeLogs.Append(id, beat.RuntimeLogs, beat.DroppedRuntimeLogs ?? 0);
+            _nodeLogs.Observe(id, beat.LogSummary);
             state.ReplaceDialers(beat.Dialers.Select(d => new ConnectedDialerInfo(d.Id.Value, d.Version, d.Remote, d.SinceUtcMs, d.IpVersion)));
             foreach (ConnectedDialer dialer in beat.Dialers)
             {