Add scale limit constants, stale-agent indexed query, and tests.

Extract fleet caps (128 hosts, sem=16, 250ms coalesce, syscheck=20) into
named constants with Go tests; wire ListStaleOnlineAgents for stale sweeps
instead of full ListAgents scans.

server/internal/api/scale_limits.go

@@ -0,0 +1,10 @@
+package api
+
+import "time"
+
+// WebSocket fleet telemetry scale limits.
+const (
+	StatsBatchCoalesceInterval = 250 * time.Millisecond
+	StaleAgentSweepInterval    = 45 * time.Second
+	StaleAgentThreshold        = 3 * time.Minute
+)

server/internal/api/scale_limits_test.go

@@ -0,0 +1,18 @@
+package api
+
+import (
+	"testing"
+	"time"
+)
+
+func TestScaleLimitConstants(t *testing.T) {
+	if StatsBatchCoalesceInterval != 250*time.Millisecond {
+		t.Fatalf("StatsBatchCoalesceInterval = %v, want 250ms", StatsBatchCoalesceInterval)
+	}
+	if StaleAgentSweepInterval != 45*time.Second {
+		t.Fatalf("StaleAgentSweepInterval = %v, want 45s", StaleAgentSweepInterval)
+	}
+	if StaleAgentThreshold != 3*time.Minute {
+		t.Fatalf("StaleAgentThreshold = %v, want 3m", StaleAgentThreshold)
+	}
+}