feat(cluster): the spot list size is a setting

The list is a ring buffer that held a thousand spots, and on a busy
evening a thousand arrive in a couple of minutes. So the buffer decided
how long a spot lived, and the spot LIFETIME setting never got the chance
to expire anything: fifteen minutes meant nothing when the oldest spot
was pushed out after two.

Now settable (Preferences → Cluster, beside the lifetime, since between
them they decide the same thing), 100 to 10 000. The ceiling is a real
limit rather than a round number: every spot is matched against the
worked index and the alert rules, and is a row the cluster grid and every
open band map re-render.

app.go

@@ -312,6 +312,7 @@ const (
 	keyClusterSelfSpot    = "cluster.self_spot"         // "1" → announce ourselves on the cluster as we log
 	keyClusterSelfSpotMin = "cluster.self_spot_minutes" // shortest gap between two self-spots
 	keyClusterSpotTTL     = "cluster.spot_ttl_min"      // drop spots older than this; 0 = keep them
+	keyClusterSpotMax     = "cluster.spot_max"          // how many spots the list holds at once
 
 	keyScpEnabled = "scp.enabled" // Super Check Partial / N+1 suggestions on
 
@@ -20294,6 +20295,50 @@ func (a *App) SetKenwoodKeySpeed(wpm int) error {
 // and exists only so a mistyped value cannot mean "never".
 const spotTTLMax = 720
 
+// The spot list is a ring buffer, and its size decided the lifetime far more
+// often than the lifetime setting did: on a busy evening a thousand spots
+// arrive in a couple of minutes, so a fifteen-minute lifetime never got the
+// chance to expire anything. Hence a setting.
+//
+// The ceiling is a real limit, not a round number. Every spot is matched against
+// the worked index and the alert rules, and each one is a row the cluster grid
+// and every open band map re-render; past ten thousand that work starts to show
+// on the very evenings the list is worth having.
+const (
+	spotMaxDefault = 1000
+	spotMaxCeiling = 10000
+	spotMaxFloor   = 100
+)
+
+// GetSpotMax returns how many spots the list holds.
+func (a *App) GetSpotMax() int {
+	n, _ := strconv.Atoi(a.settingOr(keyClusterSpotMax, ""))
+	if n <= 0 {
+		return spotMaxDefault
+	}
+	return clampSpotMax(n)
+}
+
+// SetSpotMax sets it. Clamped here as well as in the UI, for the same reason as
+// the lifetime: the value decides how much work arrives on every spot, and a
+// stale frontend must not be able to widen it past the ceiling.
+func (a *App) SetSpotMax(n int) error {
+	a.setSetting(keyClusterSpotMax, strconv.Itoa(clampSpotMax(n)))
+	return nil
+}
+
+func clampSpotMax(n int) int {
+	if n < spotMaxFloor {
+		return spotMaxFloor
+	}
+	if n > spotMaxCeiling {
+		return spotMaxCeiling
+	}
+	return n
+}
+
+
+
 // GetSpotTTLMinutes returns how long a spot stays in the list, in minutes.
 // 0 means spots are kept until the count cap pushes them out, which is what
 // OpsLog always did.