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/fleet/src/Fleet.Core/NodeSupport.cs

@@ -15,15 +15,27 @@ public interface INodeLog
     void Write(NodeLogLevel level, string message, params (string Key, object? Value)[] fields);
 }
 
-/// <summary>One structured node log line queued for delivery to the control plane.</summary>
+/// <summary>One structured node log line.</summary>
 public sealed record NodeLogRecord(long AtUtcMs, string Level, string Message, Dictionary<string, string> Fields);
 
-public sealed record NodeLogBatch(IReadOnlyList<NodeLogRecord> Records, long Dropped);
+/// <summary>A retained log line: <paramref name="Id"/> grows with every line, <paramref name="Type"/> is a stable hash of the message text (the template), so counters can be sent without the text.</summary>
+public sealed record NodeLogEntry(long Id, uint Type, NodeLogRecord Record);
 
-/// <summary>Optional bounded log source used by node heartbeats to upload recent local logs.</summary>
-public interface INodeLogBatchSource
+/// <summary>How many lines of one type and level were written since the last summary.</summary>
+public sealed record NodeLogTypeCount(uint Type, string Level, long Count);
+
+/// <summary>What a node tells the control plane about its logs without sending any text: counters since the last summary and the newest line id.</summary>
+public sealed record NodeLogSummary(long LastId, long Info, long Warn, long Error, IReadOnlyList<NodeLogTypeCount> Types);
+
+public sealed record NodeLogPage(IReadOnlyList<NodeLogEntry> Items, long LastId);
+
+/// <summary>Bounded local log retention. The control plane gets counters in heartbeats and pulls the text when it needs it.</summary>
+public interface INodeLogSource
 {
-    NodeLogBatch TakeBatch(int maxRecords);
+    NodeLogSummary TakeSummary();
+
+    /// <summary>Lines with an id above <paramref name="afterId"/>, oldest first, at most <paramref name="limit"/>, at or above <paramref name="minLevel"/> (info, warn or error).</summary>
+    NodeLogPage Read(long afterId, int limit, string? minLevel);
 }
 
 public sealed class NullNodeLog : INodeLog