feat: add observability buffer for heartbeat data

- Introduced `ObservabilityBufferStore` to manage buffered observability records.
- Implemented methods for upserting, replaying, and acknowledging records in the buffer.
- Enhanced `AgentNodePayload` and `NodePayload` to include buffered observability data.
- Updated tests to cover new functionality for observability buffering.
- Modified existing services to persist and handle buffered observability data during heartbeats.
- Added configuration options for observability buffer path and replay minutes.
- Updated documentation to reflect new observability features and configurations.

atsf_server/service/observability_trends_test.go

@@ -0,0 +1,32 @@
+package service
+
+import (
+	"atsflare/model"
+	"testing"
+	"time"
+)
+
+func TestBuildDiskIOTrendPointsUsesCounterDelta(t *testing.T) {
+	now := time.Date(2026, 3, 14, 18, 30, 0, 0, time.UTC)
+	start := trendWindowStart(now)
+
+	points := buildDiskIOTrendPoints(now, []*model.NodeMetricSnapshot{
+		{
+			NodeID:         "node-a",
+			CapturedAt:     start.Add(22 * time.Hour),
+			DiskReadBytes:  100,
+			DiskWriteBytes: 200,
+		},
+		{
+			NodeID:         "node-a",
+			CapturedAt:     start.Add(23 * time.Hour),
+			DiskReadBytes:  250,
+			DiskWriteBytes: 260,
+		},
+	})
+
+	last := points[len(points)-1]
+	if last.DiskReadBytes != 150 || last.DiskWriteBytes != 60 {
+		t.Fatalf("expected disk io trend to use counter delta, got %+v", last)
+	}
+}